1. Fire is useless atm. Sure, the singletarget and burst DPS was too high, but did they have to make the spec so weak you don't even want to play it on cleavefights and fights where you have to move a lot? I love playing fire since Classic and with the MoP changes it was even more enjoyable... but now you can't really play it anymore if you want to compete DPS-wise.
2. Frost could be great, but because of PvP Blizzard just isn't able to make it strong enough to be more than a situational spec that you want on ~2-3 out of 16 HC fights. It's a missed chance imo as i like playing the spec very much.
3. We are forced to play Arcane and the DPS is really good (too good which means it will be nerfed soon) but i don't like the whole playstyle with being so dependant on Rune of Power and casts you have to stand still for. In times where nearly every ranged-class can move without much penalties it feels very clunky to play arcane on every fight that involves lots of movement. And when Arcane gets nerfed its even less enjoyable. Moreover Arcane is boring to play as is now lacks everything that was the "signature" of arcane: Burst.
4. Every spec needs different Stats (Frost haste, Arcane mastery, Fire crit) which means you have to reforge AND regem everytime you want to respec, which encourages to just stay arcane for all fights.
5. I have the feeling that Warlocks are the better mages on most fights which is something we already know from TBC and doesnt really feel good because warlocks bring so much more to the table utility-and supportwise.