A bloody refined version of it's predecessor.
Now we have the sequel, a game that so expertly refines every aspect of the original, that it is difficult to understand why some critics are marking it less than the original. The original was nonstop tension, which was great initially, but once your threshold for stress was elevated, the reminder of the game felt predictable and nonthreatening.
"Dead Space 2" brings the same scares as the original, but is better paced. Interspersed throughout the campaign are moments of quiet and solitude that invite the player to relax and begin to feel at ease. This pacing alone makes "Dead Space 2" a more refined version of it's predecessor.
The combat hasn't been altered all that much, but new additions (like impaling enemies on there own appendages) flesh out what was introduced in the original. Being a sequel, I wouldn't expect many drastic changes to the combat formula and cannot think of any franchise that truly alters the core mechanics.
The sound design and sparse score continue to work flawlessly. Each spark, droplet and crackling fire lends itself to creating the best atmosphere of 2011 (and puts to shame most atmospheric games since the original "Dead Space" in '08). The voice acting works great with only a few moments of inconsistency.
The graphics have been improved, which is to be expected. The facial animations and character models all look more realistic and believable. While not up to the level of "Uncharted 2", cut-scenes and action set-pieces are well made and involving.
Another improvement is the ability to fully control zero-g movement. The weakest part of the original was the inability to direct your own course while floating.
Isaac is now a three-dimensional character with a serious amount of depth that makes his internal struggle more intriguing than the over-arching plot.
If you enjoyed the original, I strongly recommend "Dead Space 2" for the PlayStation 3 (as it comes with "Dead Space: Extraction").