The Kartel has published a brief history of the System Shock and BioShock franchises while also following the evolution of the settings, characters, storylines, and mechanics that we've seen utilized in all four titles to date.
The entire mechanism of hacking in-game security was pioneered by System Shock. If you've ever manipulated a turret, camera, or vending machine to serve your purposes, you have System Shock to thank. Saving up points or currency to buy abilities? That's System Shock 2. ADAM and plasmids are the new adaptation of cyber modules and psionic powers.
In Bioshock 2, the biggest incentive to getting that precious ADAM is so you can in turn spend it on those cool abilities (plasmids) and latent bonuses (gene tonics). The rest of the gameplay elements are woven brilliantly around this focus: you kill a big daddy so you can adopt a little sister, who you then defend as she harvests ADAM, afterwards dropping her off at a vent so you can go to another area and repeat the process. It keeps you moving and using all the tools at your disposal, which both serve to move the game forward. Setting up traps before a harvest and then fending off the onslaught is wicked fun, and getting ADAM is like hitting a drug.