I'm really glad to report that everything seems to be functioning decently. Many specs have been simplified, yes, but there are a ton of options in talents to complicate what you've got going on. So far I really haven't been able to find anything as horrible as 6.0 Arms - indeed, Arms itself has gotten back some of the things that made it compelling in 5.4 and earlier, such as a proc to look out for with Colossus Smash.
It looks like they'll need to implement or re-implement a lot of the in-game power auras, but mechanically I'm pretty satisfied with what we've got - and of course our Artifact Weapons will be adding some functionality to it all.
Unholy Death Knights:
Unholy's kit is pretty different now. You're going to be focused on popping the festering wounds on your targets with Scourge Strike. The unpredictability comes from the number of wounds your Festering Strike applies to the target. Obviously the rune changes make every DK spec feel simpler, but everything flows pretty easily for Unholy now. Also, you can get an abomination as well as an additional skeletal soldier through talents!
This really feels like a bunch of common sense stuff they should have implemented during Warlords, but better late than never. Mortal Strike is your big Rage spender, but you can save your Rage for when you have Colossus Smash, which now get reset via the passive Tactician. Slam is your filler for when you are capping Rage but don't want to waste the cooldown on Mortal Strike. You still have Whirlwind instead of Thunderclap, which seems wrong, but at least the animation is way better now. Also, you can definitely talent in some more abilities, including Overpower.
This is really stripped down to the basics - you'll maintain Rupture and use Envenom while it's up, building combo points with Mutilate. Garrote's now your main opener, and there seems to be talents affecting its cooldown, so it might be that I'm missing something here. Still, a decent choice for basic Rogue stuff. I miss Dispatch, but you can find procs and such in the other specs.
I was finding a few moments of being Energy-depleted, but overall you've got some cool stuff here. I imagine it will play a lot better with my custom gun sounds, because right now you can barely hear Pistol Shot. Some of the old Combat abilities are talents, like Killing Spree. Overall it shouldn't seem too alien to those who know Rogues.
Again, not too difficult to get used to. You build up Maelstrom with LB and LvB and pay them off with your Shocks. LvB still has a proc associated with it, so it should feel pretty familiar. One thing is that Earthquake is now Earthquake Totem and now costs Maelstrom, but works pretty much the same.
You'll feel far more in control now that Astral Power has replaced the Balance Meter. Keep up your DOTs and then build AP with Solar Wrath and Lunar Strike, then pay it off with either Starfire or Starfall (single target and AoE respectively.) Picking between Solar Wrath and Lunar Strike will, I think, be based on the buffs received from casting Starfire and, if those are not up and you can't cast it yet, I think SW is single-target while LS does splash damage.
Beast Mastery Hunters:
While different, it's not too hard to wrap your head around it. For one thing, Focus seems to regenerate at the same speed as Energy now (making it basically the same thing,) so you'll worry less about actively regenerating it. I'll admit I haven't spent as much time on Hunters, so I might revisit this.
A lot of the same tools are here - Vampiric Touch, Shadow Word: Pain, Mind Blast and Mind Flay (which can be talented to be Mind Spike.) The main thing is that now you have Void Form to play with. I suspect Haste is going to be huge for Shadow, as you'll want to pump out as many Insanity-generating abilities as possible in that window.
If there's one thing I'm overjoyed about, it's that Breath of Fire is now a real part of your rotation, basically taking over for Blackout Strike in AoE situations. Without Chi, you'll be dealing with simple cooldowns, similar to Protection Paladins. As always, it's tricky to test a tank when not running challenging group content, but rotationally I think we've got something that could work.
Beyond tracking Arcane Charges in the UI (finally,) Arcane plays very similarly, though the new Mastery could, I imagine, affect how one goes about burst damage.
Like Arcane, they took a "if not broke, don't fix" attitude to the spec (and given that they're giving Frost a replacement for Frostfire Bolt, it looks like they've got that attitude toward the whole class.)
One thing I'm definitely thinking about is that everyone's basically tri-specced now (Druids are quad-specced and Demon Hunters are dual-specced, but you get the idea.) Thankfully, it's going to be much less of a pain to gear, as outside of special trinkets and items with role-specific procs or activated abilities, it's really just weapons that you need to switch out between specs. Granted, it does kind of make Strength, Agility, and Intellect sort of meaningless, or at least makes them all equivalent, but perhaps that's a price we can pay to make switching specs less of a hassle. It certainly means that people are going to be able to use gear that drops in raids. Just means everyone really needs to be on top of what their best secondary stats are.