WARNING — A FORMIDABLE ENEMY "YMMV TROPES" HAS ENTERED THE BATTLE
Complacent Gaming Syndrome: A lot of players will stick to the Field and Variable weapons upon unlocking them due to their power and range. This is especially true of the Field weapon, who's ease of use makes the game a breeze. Since the leaderboards aren't segregated by ship, using either of them is a must for chasing the rankings.
Game-Breaker: Nagoya Attacks are incredibly useful when racking up millions of points due to how they work. If you can run into a rapid-firing boss and know their attack patterns, it will usually be better to just farm Nagoya Attacks until the boss leaves than to actually kill the boss in the first place.
Genius Bonus: Stage names tend to be related to what additions that stage brings to the game, or what changes will be made to enemy behavior. For example, the very first stage, Common Descent, has you playing a basic round of Space Invaders where all changes to the formula and enemies will originate from.
Good Bad Bugs: In the PlayStation 3 and Xbox 360 releases, the "difficulty clear" achievements don't actually require you to do a complete run of the game; you just need to beat the final stage with the difficulty set to the corresponding level, making it trivially easy.
Nightmare Fuel: The Final Boss and the leadup to it, pushing Nothing Is Scarier to the max. The level begins with your ship traveling through an unknown location with near-zero going on, an echoing siren in the background accompanied by odd, intermittent static. You approach three sets of gates, which open slowly and force your ship into exactly the middle of the screen. Eventually, the camera begins to pan behind your ship, highlighting the fact that there's nothing anywhere, in any direction. With little leadup, the boss warning plays - "A formidable enemy, Nameless, has entered the battle." The boss looks completely different from anything else you've encountered so far - it's just a black circle with a white outline, pulsing slowly unlike anything else in the game. Unlike any other boss, this one has the capacity to kill you almost as soon as it appears unless you immediately start shooting to deflect its screen-filling attack.
Killing it results in a Sensory Abuse laden Jump Scare, pulsing wildly before the "level clear" wave comes over, reverting your ship, taking the entire game with it. The game ends back at Stage 0, with you shooting the last invader before the credits roll.
For the most part, enemies that are spawning will usually have some kind of visual indicator, such as a path or an outline of them being formed. Sometimes, however, there will be no such luck and you'll get blindsided by a tiny enemy moving into position at breakneck speeds with zero warning.
When fighting The Sun, some barriers will spawn at the beginning of the fight to protect you from its wave attack. If you lose them, you'd better hope it's running low on health; the attack will become almost completely unavoidable. Hope you have enough ships in reserve!
The amount of ships you start with is all you've got - there's no way to get any more without running out of them and getting a Game Over. Have fun with the boss rushes when you only have one ship left!
Unless you're already shooting, the final boss can kill you the very moment it appears on screen due to how its attacks work. Hopefully that wasn't your last life that you just lost!
The Rapid weapon. It's weak, can't shoot diagonally or behind you like many other weapons, and has trouble getting through even the weakest enemies - even at full power.
The Option weapon is even worse; the bolts it fires are nothing to write home about, and managing your Options is quite difficult given the amount of movement you need to do - or lack thereof, in some cases. More to the point, it fires relatively slowly and its wide range of fire at full power can be more easily attained with other weapons, such as the Variable.
That One Achievement: Continue-Free Clear, which requires you to complete the entire campaign, from the very first stage to the last without running out of lives and in a single session. Even on Easy with nine ships in stock, this is no simple task.
