Spoilers:
Well, Archie thought they didn't really give William a fair showing in the end of the movie. Michael had no harder time fighting him than he did other werewolves and William was supposed to be the uber werewolf. He had time to feed after being released and that would have been all he needed to be badass in no time.
Also, Marcus, centuries old warrior who seriously would forgotten more about fighting than Selene could have ever learned in her times, let's himself be turned around and have the big slicing blades put behind him? Yes... this would not throw up red flags to any smart warrior. Nope, he wouldn't be worried at all. And previously he's grabbing and tossing people with ease. He'd have picked her little leather clad butt up and tossed her in those blades before she could blink but nope, he stands there and let's her hit on him for awhile...
Also, they made a big showing of the fact that those little hand bombs looked exactly like the amulet. Yet, after having set it up, they never used that fact AT all.
They also never fully explained Alexander Corvinus. He comes off as looking like just another vampire but you have to look closely to both movies to realize he wasn't. He was immortal, that was the weird gene he possessed, but his sons were bitten by bat and wolf... Although, with them being Bulgarian, or at least from that region, I want to know how Marcus got bitten by a bat that consumes blood when the nearest species for that is in South America....
But other than that, it's cool movie.