A few things, but one word in particular comes to mind: Immersion.
This is the same thing that made Half Life 2 so popular. The second you leave the train station and step into the plaza, you are there. You are standing in the middle of City 17, you are a part of that world. You must learn what it takes to survive there. The overall environment and story is much different than the first Half Life.
Bioshock, pulls this off just as seamlessly, though with a much different atmosphere. You get in the bathysphere not knowing exactly what to expect. You see a few Art Deco sculptures on the way down. Without being pulled out of first-person, you're treated to a bit of Andrew Ryan's propaganda. You learn what he wants, why he wants it, and what Rapture's purpose is. Then the screen pulls up and... WOW! Then when you surface in Rapture, the sense of wonder you felt on the way in quickly turns to dread as you realize that you are in a deteriorating underwater hellhole, and a bunch of drugged-up psychopaths are trying to kill you. You must learn to survive there very, very quickly. This also plays for a brilliant mood whiplash. Also, the story is well told, the combat can be wildly varied depending on your mood. [http://tvtropes.org/pmwiki/pmwiki.php/Main/VideogameCrueltyPotential] (You can be very sadistic to splicers if you feel like it.) This also creates replay value.
Now compare this to Bioshock 2. Not much seems new. Rapture is under the control of an ideological extremist... just like the first game. The levels are different but the atmosphere is the same. You've been here before, you know what it takes to survive. Nothing seems that novel, save for the fat that you play as a Big Daddy, but that actually makes things worse. While it might feel satisfying to be able to curb stomp some enemies, it's much harder to relate to Subject Delta, a splicer slaughtering behemoth, than it is to relate to Jack, who is one very scared individual simply trying to survive. Jack's situation is more along the lines how most of us would feel in that scenario, and Bioshock 2 throws that feeling out the window.
And once again I've written way too much on what should be a short subject. I do have high hopes for Infinite, and am crossing my fingers that Levine will come through for this one.