Still on my first play through, but some general impressions:

Fairly solid shooter/plasmid game. Just the fact that you can use both simultaneously at any point makes for less hassle. While I thought I'd lose that "OMG OMG SWITCH", they seem to up the amount of action to accommodate the tension there... maybe.. can't remember the first game that much. :p

I like that they've opened up the gene tonics by not restricting you to different classes, and they've increased the number of slots at the same time. Sure it's more mainstream, but there are a ton of tonics I find useless and don't want to be restricted in that manner...

Definitely enjoying the differences between the upgraded plasmids, but it seems cumbersome during battle with the charging. It's something to get used towards with the timing, which I don't like.

Hacking seems... out of character for the story (a moving needle), but it's alright. Faster pace.

It doesn't have the same mysteriousness as the first, though I'm finding the story very well executed as you almost constantly get audio recordings & exposition from main characters.

I sorta miss some of the spookier settings in the first game. For example, there was a giant room with no lighting except from outside via massive glass windows comprising one of the walls. Place is flooded. As you explore the room (there was a tonic to compel your exploration), you can't help but feel something is wrong with the dead silence. And some statues are seemingly moving instantly when you turn around. Makes you think... ok.... check the room out again. Then you notice there are more "statues" creeping up on you. >:D And wham, attack!

So yeah, Bioshock 2 feels very different, being a big daddy instead. Less fear, more... being kick ass I guess. It might seem a little weird with splicers trying to attack you instead of fleeing, but there is a story reason.

Graphically, the game in general is much more well lit. There are dark areas where you get an auto-flashlight, but it doesn't feel the same for some reason. So far, the tech seems to be relatively unchanged, which is a bit of a downer... character models are better on the upside. Water tech should have been improved, but it can still look good.

Wish you could back track to previous levels to explore, but I guess the story is not designed/executed to do that. That said, the ocean walking is rather bland and somewhat pointless so far. Kinda miss seeing big daddies repairing stuff like we only see in the intro of the first game. :s Just thought there'd be more things to do outside like that.

Saving the little sisters gets a bit tedious after so many straight up battles. Would have been interesting if a roaming big daddy could steal her from you. :p

Multiplayer: worthless. Freeze. shatter. or... get wasted by big daddy player. No one seems to play team games either. so... meh. waste of contract money. And screw the multiplayer achievements. :p