Awesome Art: Oh yes. The artwork, after some Art Evolution, is simply stunning; it's painstakingly detailed and brimming with atmosphere in every single frame. In particular, the intricately detailed Mayan statues in 32 Chambers, the beautiful bioluminescent plants in The Core, the mosque in The Plan and pretty much the entirety of The Temple and The Exit stand out.
The baroque-style organ music on the lowest level of The Temple when you discover the tombs of Murtaugh and Liz.
The hard breakbeat track used for the beginning of The Loop, for some. It returns in the Legacy collection as the theme for the 'Break the Loop' menu, where it now represents the act of mastering and subverting the most trap-like part of the complex.
Epileptic Trees: The game tends to support fan theories, even having a page dedicated to theories that eventually got included in their own "game", Submachine Universe.
Funny Moments: In Universe there's a note at the bottom of the ladder in the chasm* coordinates 613. While quite unnerving due to being vaguely nonsensical, one of the sentences, reading "Get this thing off me! Get it off!!" is very hard not to associate with Black Mesa scientist being humped by a headcrab. It shifts the mood from creepy right into hilarious.
Heartwarming Moments: At the very end of the series, after thirteen games of empty rooms and cryptic notes you escape the Submachine and see two people. One is a woman. The other is a man with a karma arm. And there's a black cat in the background.
Sometimes in later games, you'll be able to visit places you've been to in earlier games and find that things have changed. Some of these changes are because of your actions when you were there before, but then there are... other changes, made by someone or something else, that are not explained. Like smashed lights, broken ladders, added karma portals, and coordinates 747's rubble cleared away to reveal a second corridor that seems to loop forever. The likely real-life reasons for these changes (practical and/or Rule of Cool) may or may not make this any less creepy.
One theory* coordinates 128 in Universe suggests that the Subnet may have been changing the letters you find to make the previously benevolent Mur seem evil. After he apparently leaves you to die you still escape through a very convenient portal that gives you easy access to the Core.
Because what [the Subnet] might have been doing all along is replacing its old defence system with a new one. One which is mobile, unhackable, highly skilled and capable of infiltrating the human infestation. One that has been conditioned to think that Mur is a traitor. You.
This can be switched to a second theory where certain key words are changed — and Mur has been conditioned to hunt down Liz.
Player Punch: In Sub 9 Murtaugh, the guy who has been directing you, manipulating you, speaking with you, and who had been your only likely chance for a human connection in the Subnet, is found dead. And not just dead. Mummified. Who knows how long he has been dead? You're alone and always have been. And the same case for Elizabeth a little later on...
Sequel Difficulty Spike: Each game can be described to be more challenging than the last, but as a result of 10 being bigger than the first nine games combined, paired with extremely complicated puzzles and more disorienting navigation, it is much harder than any of the previous games.
that eventually got included in their own "game", Submachine Universe.