The purple worms that suck you in and kill you
Good God yes, those enemies were annoying as all hell.
Personally I thought most of the DLCs were pretty forgettable and not worth a purchase. I just bought Tiny Tina's DLC yesterday on sale though, so I haven't played that yet to give an opinion.
If When Gearbox releases BL3 and starts to create DLC for it, if it is being outsourced then I won't bother purchasing it not even when it goes on sale. Complete waste of time and money.
The additional characters are amazing though. I am currently leveling a Mechro and having tons and tons of fun with her. Haven't tried Krieg yet but he seems like a fun character to play too.
I also hope that they come up with a better way of handling TVHM and UVHM than just replaying the campaign over and over again. It works for games like Diablo 3 because of how quickly you can blast through the campaign, but it takes ages in BL2 and the difficulty curve can get pretty steep if you're not getting lucky drops.
I also wish the bosses and high-end content were more easily solo-able. Yeah it's fun to get 3 friends and attack this content, but most of the time I'm playing solo and I can't be bothered to dedicate the time required to cheese some of the boss fights.