I understood that Bioshock 2 picked up from the point where you became a Big Daddy in the original Bioshock, but I don't know if that's such a good point to pick up from or not. My brother suggests its story-line is rather separated from Bioshock 1 in how whoever Bioshock 2's antagonist is was never really mentioned in BS1 and that you explore much more generic places than where BS1 had you see, but the tonics/plasmids still advances from the original too.
I'm somewhat in KingButz's boat about what to play next. I'll probably play Phoenix Wright: Trials and Tribulations just so I can finish that trilogy, but nothing is certain right now.
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http://img812.imageshack.us/img812/3492/eponaharr.jpg
Nominate SuperNiceDog's Guru-choice, then Epona.