Dead Space 2 adds upon the original in almost every way with only a couple minor issues.
Dead Space 2's gameplay is identical to the original in most respects. It still takes on that over the shoulder shooter style made popular by Resident Evil 4. You have all the weapons from the first game along with some new ones like the javelin gun and rivet gun. There are also new monsters to face like the puker, whose vomit slows Isaac down. For the better part, everything is the same. There are a few changes that I feel are much welcome. Zero G moments allow you to pilot your suit as opposed to just jumping like in the first one. I feel this replicates zero gravity much better. You're also going to have to hack panels in a minigame at certain parts of the game. These minigames aren't terribly hard or entertaining, but they are different.
Visually, I think Dead Space 2 is a step up. The characters look a little better and the environments are much more detailed. However, I feel some of the terror is gone because the environments are much more open as opposed to the confined space of the Ishimura (which you get to visit during one part of the game). The environments also maintain freshness in the fact that there is no backtracking in this game. You're not going to be forced to go back into the apartments for some silly objective like the first game. However, this game does feel a tad more linear because of it.
The sound design for Dead Space 2 is much improved as well. Isaac is no longer a silent protagonist. He talks, gets angry, yells, etc. The other characters in the game sound suitably realistic, especially the gibbering lunatic Stross. Music is the same intense, tension building sounds like the first game.
There's an immense amount of value to this game, far outstripping the first. First, there's Dead Space: Extraction. This was a Wii exclusive that you can play with your Playstation Move (or controller, but I don't advise this). The game has its own trophy set and comes free with the Playstation 3 version of Dead Space.
Dead Space 2 also has a multiplayer element to it. You can either choose to be humans or necromorphs and battle it out. The necromorphs deserve particular mention as each has specific abilities to help out the team. Dead Space 2 also has several good DLC items, most notably the Severed package. This standalone adventure takes place at the same time as the regular game with the characters from Extraction tying everything nicely together.
Dead Space 2 is a rarity indeed. It's hard to find another sequel that builds so nicely upon the original with as much content as this game has. Even after one beats the game, there are several trophies that will require a second playthrough to obtain. You'll also have a good amount of content to gain from Extraction. If you are looking for a game to invest plenty of time in, this is it.
The Good: Builds upon the original in almost every way, excellent graphics and sound, new necromorphs require special tactics, good DLC, Extraction included for free, multiplayer increases replay value.
The Bad: Open spaces reduce the terror some, for better or worse largely the same in gameplay as the first.