I picked up BIoshock 2 on release day and dove into the single player.  After getting tired of having my ass handed to me by Big Sisters, I decided I'd test out the multiplayer.  Understand that prior to the games launch, I was in the camp that was so mad that the game had multiplayer.  I loved the original and having multiplayer in the franchise seemed almost sacrilegious.   19 hours 45 minutes later I hit level 40 and can finally jump back into single player.   
 
I should set up that I'm not a big multiplayer gamer.  I don't play MW2 or Halo 3 or anything online, but for some reason the multiplayer of Bioshock 2 felt satisfying. 
 
What I would change... 

• Nerf the nail gun.  As someone who used it once I unlocked it all the way up to level 40...that gun is mad powerful.  While zapping someone with electricity is a good counter measure, once getting into higher levels I seldom saw many people using it.  The fact that it's great at short-medium range made it a must use weapon.
• Buff the tommy gun.  There is no reason why the tommy gun should be less effective then a nail gun.  On top of it not requiring the rev up period the nail gun has...the concept that a nail does more damage then a bullet is mind blowing.  The problem is that the tommy gun is classified as a medium range weapon so unless you're at medium range, which isn't clearly defined,  it's effectiveness is decreased.
• More spawn points for Big Daddy suits.  Self explanatory.  I can't think of a reason each map only contains 2-3 spawn points for the suit.
• Wider selection of maps.  Yes I get the theme they were trying to accomplish with the maps and I get that Rapture is a wet dark place under the ocean.  But all the maps felt the same.
• Easier ways to kill the Big Daddy on lower levels.  The nail gun rips through the Big Daddy however at lower levels, anyone who's half way okay at the game can roll through people in a Big Daddy suit.

• Random lockup issues.  I could almost bank on the game locking up at least once an hour, if not more.  No rhyme or reason why but this needs a patch pronto.  One time i completed a pretty difficult challenge and the game locked up in the post game score screen (before returning to the lobby).  I booted back up the game and didn't record the credit credit...so 10 minutes and 1500 ADAM lost forever.
• Lag.  I've been playing the game for a week and while it's gotten a bit better, still experiencing lag issues which just makes the experience feel amateur.  

• Balance.  No matter how powerful something was there was always a counter.
• Big Daddy.  It was a nice touch.  Many times in team matches the Big Daddy suit resulted in a huge momentum shift and proved to be a real X factor.
• The Apartment.  A nice little hub that tied together the lore of the franchise and mutliplayer together really well.  The sequence that occurs once hitting level 40 is also pretty slick.