As you can imagine, playing as a Big Daddy changes certain aspects of the game, but the core shooting mechanic is more or less still classic BioShock. The sequel throws at you a more defensive aspect of the game as you face an onslaught of splicers while defending a Little Sister that harvests Adam from a corpse (think of Adam as experience points for upgrades). As far as the story goes, you are out in search of your character's Little Sister. But in the meantime, you can kill other Big Daddy's and take their Little Sister's. You can choose to rescue them or harvest them just like in the last game.
BioShock 2 is heavy on action. You get thrown into tight situations quite often and will have to quickly fend for yourself. Thankfully, you have tools such as the hack dart that allow you to take control of enemy gun turrets, cameras and flying attack bots. Hacking them turns them onto your side against Splicers and other enemies in the game. In fact, you can hack almost anything, from doors to vending machines (for discounts on weapon ammo). You can also pick items (such as ammo and money) off of corpses, from objects such as desks and by buying them in said vending machines.
The developers put a heavy emphasis on both weapons and on powers. My personal favorite gun is the shotgun, but you will eventually have variants on rocket launchers, spear guns, machine guns and so on. Each weapon has three upgrades that allow you to improve damage as well as abilities such as electricity. You can upgrade these at weapon upgrade stations, which are free, but only allow you to choose one upgrade per station.
Not satisfied with single-player alone, the developers included a multi-player mode to BioShock 2. I'm still not sure if this was a wise choice or not, but I can definitely say that they did not sacrifice the quality of the single-player experience to add a multi-player mode. That said, there are not a ton of people that play online, especially for the PS3. It can be entertaining for a few hours, especially the Capture the Sister mode.
Fans of the original BioShock definitely need to check the sequel out. The single-player experience will last you a good ten to twelve hours, depending on your difficulty setting, and the multi-player adds at least a few hours of enjoyment. It's a game that I did not expect to perform on par with the first, but somehow it managed. A few different endings make playing through at least twice (once as good and once as bad) worth the effort.