Halo05 said it best, Bioshock 2 didn't have the "wow" factor going for it because we've already experienced Rapture, but that's hardly a reason to knock it. It was a fantastic sequel and definitely underrated.
I wouldn't say Bio2 had monster closets but I think it definitely had more areas where you could play with your weapons, plasmids & tonics, set traps and try shit out on enemies. I think Bio1 had such a strong difficulty curve that once you found a weapon/plasmid combo that worked you stuck with it. There wasn't really a reason to try out different options.
All that being said, I think Minerva's Den is the best DLC since DLC hit this gen.
I own both and all the DLC and will still probably buy this.