Mondaiji-tachi, obviously because it fell so short.
Haganai, because it's left up in the air at the point anime ended, although that next season won't happen anytime soon.
Hozuki no Reitetsu should have another season, and I don't see why it shouldn't.
There are a lot more characters that have yet to be animated, and there's apparently an allusion to Greek mythology! Comedy/episodic anime seems to be easier to work with so hopefully this wish will come true.
I know it is already confirmed but I’m so glad that Kuroko no Basket is getting a third season. KnB totally deserves to have another season and I can’t wait for it!
To be honest, after how awesome season 1 was I thought a second season was going to be automatic but I guess not. It both needs and deserves a season 2 though. It also always seemed like a series with a lot more story to tell and adventures to be had.
No Game No Life
One of the best, surprise, series all year! It's even got an ending that implies a renewal... they aren't going to make us wait till next year are they?
Valvrave the Liberator (fall 2013)
Obviously because damn plot hole, rushed ending. Altough it seem already turned into sh*t before the ending, at least season 3 can fix it, or I hope so