The Japanese boss theme samples "Work That Sucker To Death". In Quartz Quadrant, you defeat Eggman's conveyor belt contraption by running the belt until the console platform Eggman is standing on wears thin and he's forced to flee. In other words, you literally work that sucker to death.
In regards to the music, both Ogata and Nilsen were in the right mindset for the musical theming of the zones.
Palmtree Panic's tropical feel: The closest to the tropics that Japan can get is Okinawa, which explains why the American soundtrack now sounds like it's from the Caribbean.
Collision Chaos: Both have an urban feel to them but implemented differently. The Japanese soundtrack does it like it's Akihabara, while the American soundtrack does it like it's either Chicago or New York.
Tidal Tempest's watery feel to it is implemented a bit differently in both soundtracks. The Japanese soundtrack has the ocean waves and an ancient feel to it while the American soundtrack uses the ancient feel for the Bad Future, but it still has the watery feel to it with the slap bass sounding like drops of water.
Quartz Quadrant has a sort of rocky feel to it. This is apparent in the Japanese soundtrack, while the American soundtrack took rock as rock and roll.
Wacky Workbench has an industrial feel. This is heavily reflected in the Japanese soundtrack's bad future with the warning siren going off, while the American soundtrack has it sound like a building is being put up.
Stardust Speedway is a bit interesting as the Japanese soundtrack does it like a bright night on the road, while the American soundtrack has it sound like Sonic is on a racetrack, and adds a bit of foreshadowing to his eventual race with Metal Sonic.
Metal Madness both involve a techno/rock feel to them. The Japanese soundtrack includes the late Casey Rankin chewing out Eggman in rap form that all of his plans have failed and his ultimate weapon has fallen. The American soundtrack meanwhile is heavy on the techno in the Present and Good Future, while heavy on the rock in the Bad Future, showing that Eggman is starting to descend into insanity.
Similar to the above points, the Japanese and American soundtracks could be considered to represent different parts of the game... from Sonic's view. The Japanese version, being in your face and upfront about the moods of the Zones, are what Sonic sees around him, as a sort of musical version of Empathic Environment. Metallic Madness' Bad Future shows it best, with how it sounds utterly demented, and a robotic voice (implied to be Metal Sonic or Eggman) speaking bluntly about how he's won, and that Sonic has failed utterly. The American version, opting for more atmospheric and subtle melodies, especially in the Bad and Good Futures, represent his focus as he runs through the areas. This is especially apparent in the Good Future of Stardust Speedway, as it feels like he's revving up for a race against Metal Sonic and is ready to take on the remaining challenges to save Little Planet; and the Good Future of Metallic Madness, where it still sounds mostly like the Present version, but more low-key and subdued, signifying that while he still needs to deal with Eggman, Sonic is affording himself a chance to relax a bit, as he's finally won for real and just needs to deal with the man himself.
There is a reason why Eggman's robots in this game are all bugs. Unlike in previous games, where the good doctor had avian, reptilian, amphibious, and mammalian Badniks, as the game deals with time travel, having a monkey Badnik in the age of the dinosaurs would cause some cases of Fish out of Temporal Water. Bugs meanwhile were around before most chordate animals and are still alive today.
Further Brilliance: Why are the Badniks in CD using living cores that sprout into flowers when shattered? Simple: Plant seeds can remain bioelectrically viable for far longer than animals, so Eggman used flower seeds as the bioelectrical cores of any badniks he would be deploying for the extended time periods from Past to Future. They're also easier to harvest than animals, and even if the resulting badniks turn out less powerful/intelligent than the traditional animal-powered ones, he can field many more of them at once, and store them for long-term deployments without needing to constantly replace the 'cores' of the badniks due to the small animals dying from old age.
Which explains why enemies like the Kama-Kama were laughably weak compared to its Demonic Spiders successor, the Slicer from 2.
Each Round's Present version gets increasingly industrialized and urbanized as you go through the game. Sonic starts in complete wilderness; the next three Rounds are a mix of urban and rural (outskirts casino, ancient ruins, mining facility), and the last three Rounds are completely man-made in setting (factory, city, robot fortress). Perhaps this is a subtle sign that conservationalism was already losing out on the Little Planet before Sonic even got there, and it took Dr. Eggman's presence as a warning for what might happen.
Each boss, with the possible exceptions of Palmtree Panic and Metallic Madness, use the zone's gimmick: Collision Chaos has many pinball flippers lying about and the boss is a giant pinball table, Tidal Tempest's second phase takes place entirely underwater, Quartz Quadrant's has conveyor belts all over and the boss zone features one that functions as a treadmill, Wacky Workbench uses the electric floors to bounce the debris up to Eggman and Stardust Speedway is the most speed-oriented zone in the game; Naturally, its fight is a race against Metal Sonic.
Fittingly, Sonic Mania's Metallic Madness uses the shrink ray gimmick in its act 2 boss fight, leaving Palmtree Panic as the sole exception.
Why does Eggman seemingly commit Boss-Arena Idiocy by having air bubbles in his Tidal Tempest fight? He has nothing covering his Eggmobile to help him breathe and he's underwater like Sonic too, so he's effectively running his own bubble shield in a literal sense.
A similar design oversight happens with the Quartz Quadrant boss, where it would normally be a rather foolproof plan: trap Sonic in a room he can't run out of with a conveyor belt, drop spread bombs on him, and lock the exit with spikes and his wall. The big problem? He put his own controls inside the wall, and it's actually touching the ground. That conveyor isn't just running passively like the rest, it actually only moves when Sonic is running on it — Eggman underestimated how fast Sonic could actually push the conveyor and it grinds his wall up.
Why do the Past versions of zones have the same music in both the Japanese soundtrack and the American soundtrack? Because you can change the present and the future, but you can't change the past.
While the method by which Sonic activates time travel (running really fast) is likely due to Super-Speed being his signature power, it also evokes another franchise in which moving a certain speed is required to travel through time. This is Lampshaded in the Steam remake where the achievement for time traveling for the first time is called "88 miles per hour", and Word of God has outright stated that the core concept and method for the time travel is directly inspired by Back to the Future.
Why is Sonic's Spin Dash is animated differently from the other games? He was just starting to develop the move. Also, while the Super Peel-Out technique may look cool and increases your speed, it was mostly impractical as there was a good chance of colliding into your enemies without any means of offense other than rolling.
It seems odd that the third Zone in every Round is what instantly forms the Good/Bad Future as soon as you arrive, whereas the previous two Zones all start in the Present. Especially with how, in the first few Rounds, there's little to no sign of any tech. However, this is a good indicator of Eggman's influence in the past having gradually more devastating consequences in the future if Sonic doesn't intervene — in Palmtree Panic, he's already gotten a head start, however little, just to test the waters and see what he can get away with. By the time of Metallic Madness, he's already in the process of constructing his final base with little sign of what used to be there before he arrived. Sonic's actions end up being the deciding factor in whether Eggman achieves what he desires, or is stopped before the ball gets rolling.
There's also a reason why getting all the Time Stones guarantees Good Futures from then on, even if you missed all the robot generators beforehand. Sonic can use the stones to make it so that Eggman's efforts are immediately undone, without needing to personally intervene. He still needs to deal with the man in the future, of course, but with the Time Stones, he doesn't need to go out of his way to ensure Little Planet's prosperity — he just has to catch up to his nemesis.
Alternatively, Eggman hasn't already collected the Time Stones yet in the Present, but will do so in the Future — meaning collecting them now prevents Eggman from ever getting his hands on them, which ensures he can't then use them to Make Wrong What Once Went Right and preventing the teleporters from existing in the first place. Hence why collecting them all is an Instant Good Ending Condition, even if you never once travel to the Past.
Fridge Awesome, really, but her presence in the final cutscene implies that Amy managed to slog through Metallic Madness all on her own and catch up with Sonic just in time for the escape sequence. Which warps around back to brilliance when you realise that that's the first indication of her Determinator status and ability to keep up with Sonic in the entire franchise, well before Adventure.
Why are Dr. Eggman's boss fights in this game so simple compared to other games? Because Eggman was looking forward to seeing how his greatest creation Metal Sonic would fare against his nemesis, which he obviously would need Sonic alive in order to do so. So while he still has the obligatory fights with Sonic, he's deliberately holding back to ensure Sonic will make it long enough to challenge his robotic rival. After all, it wouldn't be satisfying to Eggman's ego if he couldn't prove his creation was superior to the original Sonic.
This also explains why the final boss is so underwhelming. Unlike Sonic 2 and Sonic 3 & Knuckles, Eggman didn't have a backup plan to stop Sonic with, instead hinging his entire plan in CD around Metal Sonic. So when Sonic manages to be victorious over Metal, Eggman was caught completely by surprise and was forced to hastily throw something together last minute, resulting in a silly-looking mech that goes down in a few hits.
As for the instakill laser? It's Eggman's way of forcing Sonic to race to the death against Metal Sonic. He's deliberately punishing you for losing or making no attempt to race like he wants. Notably, if Sonic is hit by the laser before the race begins, he simply loses rings like normal.
If Eggman has a laser that is capable of instantly killing Sonic even if he has rings, why hasn't he ever used it before or since? As the doctor says in Sonic Unleashed, where's the fun of taking over the world without any challenge? He has the means to kill Sonic any time he wants, he just chooses not to, because he doesn't want to simply kill him— he wants to completely and utterly break the hedgehog and let him know he failed before finally going in for the kill.
If you idle for 3 minutes, Sonic snaps and jumps off the screen for an instant Game Over. This can be interpreted as Sonic being Driven to Suicide.
This should be taken with a grain of salt, as obviously this is non-canon, and was likely done for humour. Out of spite and boredom, Sonic got tired of waiting for the player to move him. In his idle animation, you can clearly see him looking at the screen with an impatient look on his face, staring at an imaginary watch.
The Bad Futures, horrifying as they are already, become even worse when you realize that the scale of Eggman's plan means that he is rendering an entire planet uninhabitable.
There is also the implication that, with every successive Round, he's falling deeper into Sanity Slippage. It's already bad enough with Palmtree Panic's Bad Future, where it's left a polluted quagmire, but it's still recognizable as what it used to be. Tidal Tempest's Bad Future ends up looking like another area entirely, like some sort of huge sewer instead of an underground water cavern. Metallic Madness' Bad Future may be the point where Eggman finally broke completely, and it shows in how utterly wrecked and broken everything is, with even the enemies just barely functioning. Achieving victory made Eggman grow mad with power and it had a ripple effect over his whole empire.
Eggman has a very childish disposition, and children can get bored quite easily. The bad futures being in ruins could be because this is some time after his conquest was successful, and he’s now growing bored with the place, and getting ready to move on to conquering the world. (And of course, there’s a good chance he would get bored with the world eventually and move on to another planet afterwards)
