These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
A good majority of the soundtrack, thanks to the musical talent of Chris Vrenna (who now plays with Marilyn Manson). The best example would have to be either "Village of the Doomed" or "Flying on the Wings of Steam", the latter of which had a remix that was put on the album.
Goddamned Bats: The Snarks that jump out of lava to spit fire at you. They're very hard to kill without homing weapons like jacks or cards since lava is life-draining to the touch. The regular Snarks are pretty annoying too, frankly.
Game Breaker: Once you get the Jacks, they'll pretty much be your most-used weapon. That is, until you get the Jabberwock's Eye Staff, which works wonders on larger groups of enemies.
The death of the Cheshire Cat is sad, but the weird squeak/meow noise he makes when it's happening is hard not to laugh at.
That One Boss: The Jabberwock in the second battle. He attacks from the air, directly from above you (which is nasty enough in TPP shooter) and his primary attack sets you on fire, causing Alice to flail and scream.
Rated M for Money: Apparently, Alice's insanity means violence, bloody decapitations, sorts of stuff Alice probably wouldn't have imagined by that point in her life (considering she was catatonic after all), and the ads seemed to merely show that this is not the family friendly Alice in Wonderland that Lewis Carroll gave us. Apparently, Executive Meddling is to be blamed for this, since American McGee himself has stated in an interview that the developers were pushed to make the game a lot Darker, Edgier, Bloodier and Gorier than they had originally envisioned it. But then again, this is not necessarily a bad thing.