I played this for about 4 hours yesterday, and while I certainly see where some of the criticism comes from, I think it's over the top.
Gaming for me is about the experience I get, the extent that I delve into this alternate reality. Without that, a game doesn't get hold of me and doesn't have me coming back.
For instance I thought the much hyped setting in Dishonored was a bit disappointing, and just played it for the sheer entertainment and gameplay-value - which was still fun, but not something I'll look back on in 5 years.
But this game really do have an atmosphere, that little something that makes me want to believe what I'm playing is real. It's not any one part of the game, not the gameplay which I agree is not particularly innovative (nor do I think it needs to be in every game I play) it's not the graphics, although it does look pretty - it's the story and the way the other parts of the game supports it. It's also the way you slowly unravels this mistery you're suddenly in the middle of as the game starts.
Having said all that, I didn't play either of the first two Bioshock's, so my expectations, although high, weren't related to experiences of those games. For me, this felt fresh, I loved the setting, the story which I'm in the middle of is intriguing, the city is lovely to run around in and the combat does what it's supposed to do in any game that isn't based solely around it - it entertains and provides development in your skills, and acts as an obstacle for your progress.