Gene tonics have been vastly improved. Before, you had "physical" tonics, "engineering" tonics, etc, and you had to manage and balance open slots for each type. Now you just have a bunch of generic gene tonic slots, and you can mix and match any types of tonics you want. Superior!
There's a new type of splicer: "brute" splicer. He's big and tough, and likes to pick things up and throw them at you. Kinda like a L4D Tank, only not as tough. He has maybe 1/5 the hit points of a Big Daddy, doesn't hit nearly as hard, and usually carries around sixty bucks on him, making "farming" these guys much more attractive than the Big D's.
The research camera is back, and boy is it improved. No more snapping pictures while you're getting killed by a random splicer. It's a movie camera that you enable before a battle, and it's recording while you're fighting. Superior!
They changed the voices for the splicers. Haven't heard the "Jesus loves me" song yet, but I swear they enlisted the same voice actor who plays the Mucuous in the Mucinex commercials. Exact. Same. Voice.
Ammo Bandito stations are finally marked on the map. Huzzah!
Learned the Drill Dash. Very useful for whomping Big Sisters. Still died twice during my second battle though.
There's some parts where you're walking around on the ocean floor. You can pick up slugs and instantly gain ADAM.
Speaking of ADAM, it seems to flow more freely in this game. I'm up to level... 5? 6? ...and I've collected and spent probably 500 ADAM by now.
There's more moral choices than just rescuing or harvesting the Little Sisters. I'm going to have to play through again as Complete Prick and see how game progression differs.