Here's what I liked:
The stealth is simple, but fun. It's not very complex... it's mostly "run from this shadow to some other shadow" in order to sneak up on guards, but somehow it remains fun throughout the entire game. This is likely due to the variation of scenery.
And that's another thing that I liked... you're not stuck in one place for an overly excessive amount of time. The entire game basically takes place at a prison, but it never feels confining.
Also, the gameplay changes with each new locale. One may have you fistfighting, another may have you sneaking up and breaking necks, and yet another may have you running with guns a'blazing.
Finally (only because I need to head to sleep), I love the massive number of checkpoints. It's really, really a nice change from many other games.
What I didn't like:
Perhaps a tiiiny bit too short. This goes right along with the fact that you never really get a nice, long opportunity to blow up everything in your sight with your guns. If they added even just one more level/area of gunfire, the game would've been complete in my eyes.