While we might learn some new details in the Systems Panel, coming up about an hour and fifteen minutes after I write this sentence, it looks like tomorrow, on Sunday, there will be class blog posts put up on the WoW website.
Every class is getting some changes, which is true in any expansion. But highlights will certainly include:
Demon Hunters - it's a new class, so everything's new. We know that DHs will be using a resource called Fury, which should be something similar to Demonic Fury, Rage, and Runic Power, generating it via some abilities and spending it with others. Additionally, it looks like Vengeance Demon Hunters, the tank spec, will have a resource called Pain in addition to Fury. Because Demon Hunters are officially the most emo class. (Makes sense, given that Illidan "Tyrande likes my brother more than me so I'm going to go off and get demonic power; that will show her" Stormrage founded the class. Still going to play one!)
Survival Hunters - the spec that's going from Ranged to Melee. I imagine this will be a kind of rebuilt-from-the-ground-up kind of spec.
Outlaw Rogues - Yes, Combat, which has been in-game since Vanilla and has often dominated as the most powerful Rogue spec (or maybe that's just been my impression as someone who has never really wanted to play it) is dead. Outlaw will retain the big, slow weapons (their artifacts are Cutlasses - though it looks like there's an alternate look to make them more like Rapiers) but is, according to Blizzard "a totally new spec." Very curious to see what they bring.
Demonology Warlocks - The general consensus is that, with Demon Hunters getting Metamorphosis, Demonology is probably going to lose it, which means a pretty profound redesign.
Shadow Priests - Supposedly they're getting a new secondary resource (or maybe even primary?) called Insanity. No details yet on how this will work.
Lots of other specs will be changing (my thoughts are on Frost DKs) but I suspect that if the systems panel touches on some classes/specs but not all, these are probably the ones that they will. Of course, there could be plenty of unannounced changes that are just as big as these ones. Maybe Enhancement Shamans will get a new resource called "Spirit Power." Maybe Feral Druids will get a Wolf Form. Maybe Arms Warriors will literally tear off their own arms and throw them at their enemies and then have to go pick them up and reattach them before repeating (pro tip: only use this ability once before gathering your thrown arm, because it might be hard to pull it off the second time.)
There are some specs I'd be happy for them to leave untouched - Prot Paladins, Ret Paladins for that matter, Frost Mages, Assassination Rogues (maybe give them a buff or some kind of better scaling.) But sometimes just getting a breath of fresh air for these classes is nice.