Personnally I think Bioshock 2 is much better than the first...people will hate me for saying that.
1. Combat feels much more natural.
2. You can duel wield guns and plasmids.
3. Unlike the first, splicers dont get ridiculously hard to kill towards the end of the game. They get harder but not to much.
4. Less security cameras and bots.
5. More variety in enemies.
So, I played Bioschock about a year ago, and I defiantly didn't' like it as much as most people did. I still thought it was a pretty good game, but for me, it was just extremely annoying. Mainly because of all the security cameras and bots that come after you if you break something, or if the cameras see you for to long. I also didn't like how hard the enemies got towards the end of the game. Most people didn't seem to mind it but I absolutely hated it.
The story is fantastic it Bioshock 2. I actually enjoyed it more than the first game. You really feel more of a connection with the little sisters and some of the characters in the game. It feels like your decisions matter more in Bioshock 2
Another thing I really liked is that Rapture really feels more like a decaying underwater city than it did in the first, (well because it occurs ten years later than the first time you were there). The city is leaking everywhere and barnacles and other sea plants are taking over the underwater city. Overall I liked the environment and scenery more in Bio 2.
There are many new ways to collect adam than the first. You can collect it from harvesting or saving the little sisters, you can set the little sister down near some dead enemies and they will collect it, you can pick it up off some dead enemies such as the big sisters. You can also find little bits of adam in the underwater sequences, (yes you can now explore outside the city), and depending on your decisions, you can get gifts of adam, and other thing that will help you.
Hacking is totally different now. You can hack things long range with a little gun type thing that shoots out a device on to whatever you want to hack. You can still hack vending machines and anything else if you are close enough just by going up to it. When you are hacking a needle pops up on your screen and moves from one side back to the other, and you have to press A when the needle is over the green part. If you hit red you will alert the security bots, and if you hit white you get a nasty shock. You can also pay money and buy it out. So if you hated hacking in Bioshock 1, you will probably enjoy this much more. It also feels allot smoother and doesn't stop the pace of the game because you can still move around while hacking and you can hack things much faster.
There is now multiplayer in Bioshcok 2. I havn't played it to much yet so I can't fairly say to much about it.
So overall I loved Bioshock 2. I don't thing they could have made a better sequel than what they did. Bioshock 2 is an awesome game and you should play it weather you didn't like Bioshock 1, or if you loved it. Bioshock 2 is truly an amazing game.