2K prove that there are plenty of reasons to return to Rapture.
The biggest change made to Bioshock 2's gameplay comes from the fact that this time around you're playing as a Big Daddy, but not just any Big Daddy, you play as the first Big Daddy, a prototype called 'Subject Delta'. The way this affects the gameplay is that this time around it allows the player to dual-wield weapons and Plasmid s at the same time, although this can be a bit overwhelming at first (I had to fiddle with the controls until they felt right), it allows much more flowing combat.
In addition to the Leadhead, Spider, and Thuggish Splicers of the first game there are now Brute Splicers, hulking enemies that have absorbed more ADAM than their earlier mentioned counterparts, there are also Big Sisters, these are Little Sisters that have grown up and attempt to 'maintain ecological balance in Rapture', yep, this means you'll have to fight them. You'll usually encounter Big Sisters when you have harvested or rescued all of the Little Sisters in any area, these are some of the best and tensest moments in the game as they are more powerful than any Big Daddy you'll come across.
So with all the improvements they've made to the gameplay, surely the story is better too, right? Well, yes and no, there are no crazy plot twists that you'll be running to tell your friends about, but there will always be people that will argue that Bioshock stole it's whole story from System Shock 2. Bioshock 2 does have some great set pieces, but the characters are not as memorable as the ones in Bioshock, for example the main antagonist Sofia Lamb never reaches the same level of menace as Andrew Ryan. Overall the story is more linear and forgettable, that being said there is one particular gameplay twist late in the game which is a stroke of genius in game design.
Graphically the game looks pretty similar to the original with the main improvement being to lighting and textures, but the real star of the show is the audio design, I highly recommend playing with headphones on, whether you're hearing the droplets of water hitting your helmet like a spring shower on a tin roof, maniacal splicers talking to themselves in the next room, or the scream of a Big Sister on the hunt for you, you will be drawn into the atmosphere completely. They really set the bar high for sound design.
I was originally doubtful when it was announced there would be a sequel to Bioshock, as were most people, but when you take into account the improved combat, the solid story, great sound design and the still-creepy atmosphere you got yourself a worthy sequel... Oh, and it has awesome multiplayer.