I, for one, have absolutely no doubt that Arcane, as an entire spec, in both PvE and PvP is going to end up in the toilet throughout the entire MoP expansion. You can bookmark this post, screenshot it if you wish, but you will see that that statement of mine will stand the test of time.
All it says to me is that mages are in a good spot, locks were in desperate need of an overhaul (imo), from the bit I've played on the beta, mages feel pretty solid still, arcane is getting some nice mobility increases and a slightly more involved rotation than the alleged "1 button wonder" we have now. Sure we're not getting tons of flashy new talents like some classes, and I personally feel our level 90 spell (alter time) is rather...lacking, but maybe I just haven't found proper uses for it yet, but I'd rather have a solid, balanced class, than have them try to add a ton of new things and make a mess of us for the next 6 to 8 months.
Keep in mind this is all just MY PERSONAL OPINION, and people will, and are welcome to, disagree with me, in fact, I expect they will :P This is WoW after all