Every expansion brings with it changes big and small to existing classes. Specs are either totally re-worked or slightly tweaked. Warlords saw a huge cull of abilities - a success in some places, perhaps overboard in others (Arms Warriors.) But in the past we had big systems like Hunter Focus replacing Mana, the implementation of Holy Power for Paladins, and the Great Warlock Revamp that gave Destruction Burning Embers and Demonology Demonic Fury.
We've heard a few juicy tidbits about upcoming changes, but I thought I'd go class-by-class and speculate on what else we might see:
The three Mage specs are actually pretty successfully distinct right now. I don't think we're going to see anything as radical as turning one of their specs into a melee one. So actually, this is one of the classes most likely to stay the same.
Given that Demonology has drifted closer and closer to the kit of the WCIII Demon Hunter, with the implementation of DHs as a class, Demonology is going to have to focus elsewhere. This will probably mean a greater focus on summoning demons. Whether this means that our Felguards/Wrathguards will just get a much bigger buff or we'll get a more interesting couple of abilities (hopefully more of a focus on having multiple demons out, to distinguish it more from Beast Mastery hunters) remains to be seen. The other specs, I'll admit, I don't really play. With Demo nerfed in 6.2, I believe that they're both fairly popular and mechanically sound.
The real question is how Discipline Priests are going to work. In the absence of something like Gladiator's Stance, a spec can really only perform one role these days. How or whether they'll pull off making a true hybrid damage/healer caster is the big question. Holy and Shadow will probably only see a few tweaks, though.
They haven't said anything explicit, but given the changes to Hunters, I find it highly likely that one of the Rogue specs will become a Ranged spec (maybe with a Crossbow artifact?) While Combat seems to be right out, the question is whether it will be Assassination or Subtlety. Personally, I find Subtlety much more feasible, especially as Assassination is really "the dagger spec." Should this change come to pass, I think that the two remaining melee specs will probably not have to change much, as there will be more breathing room in their wheelhouses.
Druids have absolutely no problem with spec distinction, as each spec performs a very different role. I do wonder a bit about whether they'll go back to having a Balance Meter that works based on spells cast rather than simply time, as I think most people preferred the Mists/Cataclysm version.
Monks have the same advantage on specs as Druids and Paladins (and Demon Hunters, for that matter) so spec identity isn't something they need to worry about. And as such, I don't think they'll need all that much work.
Obviously, every aspect of the class is something new, so it's all change. Nothing really to talk about here until we get a better sense of their mechanics (we haven't even seen what the Demon Hunter resource bar looks like.)
So the big thing here is that Survival is going melee. But beyond that, they're also defining Beast Mastery as ranged with a pet and Marksmanship as ranged with no pet. Survival will also have a pet, so Hunters will still be a "pet class," but Marksmanship will specifically be the spec that opts out of that.
They haven't said much, but I think that they could definitely prune some of Enhancement's abilities - they seemed to be passed over during the Warlords ability purge. I only hope that if they do, they'll come up with cool ways for the various abilities to interact.
Paladins are in good shape, and kind of have been since Cataclysm. I don't really foresee any major overhauls here.
The only real big thing is that the way Artifacts work will require Blizzard to choose Titan's Grip or Single Minded Fury as the official way to play a Fury Warrior (I suspect it will be the former, as it's unique to the spec.) I know that there are people who still play Arms Warriors, and it's apparently powerful enough, but as someone who played Arms through Cataclysm and Mists and had to drop it like a maggot-covered apple (is that an expression?) after the Warlords changes, I'd really like to see it changed - either back to what it was or into something more dynamic.
The biggest thing is that Frost will now be a dedicated dual-wielding spec. They're killing 2H Frost, which is quite sad. I don't know how they'll satisfy my desire for massive Obliterate crits. I do wonder whether they will change the bonuses so that dual-wielders will favor Obliterate instead of Frost Strike, as currently the DW style kind of inevitably devolves into ignoring Obliterate in favor of Howling Blast spam. The 2H rotation is somewhat more complex and challenging (not to say that it's particularly challenging - it's just that it's three main abilities rather than two) and so I'd hope that they retain that as the standard, and then buff if they become too weak on AoE. Blood and Unholy will probably not see nearly as big changes.