I know already it will be different than old retail vanilla, even mechanically in some aspects. They are changing some things around.. I'll just add an example of changing parry cap to 14% for warriors at raid bosses, which will make raid bosses parry more against tanks, and each parry triggers increased attack speed gained by boss for a while.. which will then hit harder on tanks.
Vanilla has been min/maxed to the point where a fury tank with no shield can get healed through nearly any boss with appropriate level gear while not wearing a shield 90% of the time. The extra damage is pretty irrelevant, what is relevant is threat. I can totally see good rogues/warriors intentionally attacking from the front when they're gonna get threat capped, to increase the tank's RPS=TPS.
Warrior - slow leveler, but great to play at max level. Can hold it's own in PvP, and needed class for raiding (great DPS if played as DPS, and the only real tank). For raiding warrior is really rewarding class choice. However, you are adviced to have a different grinding alt then, as warrior is not the most effective grinder compared to some other classes.
I would add that fury is probably the most "fun"/hectic class for PVE. You can do some really retarded/fun stuff when trying to cheese for high DPS logs, like, I've very fond memories of getting world #1 twin emps every week for a month, and losing it to another warrior doing the same thing by literally just rounding up the scarabs and AoEing them down while sit spamming to keep enrage while half of our healers do nothing but turrent heal me because I have the scrab dot stacked on me to like 50. There are lots of places you can do that sorta stuff when your healers are geared enough, like sitting in AoE intentionally, tanking trash with no mitigation gear, that kinda thing.
The biggest downside to warrior IMO is literally (((like, literal use of the word literally))) every single class counters you in 1v1, and in team fights/BGs post AQ they're actually kinda garbage, too. In MC level gear, warriors are going to kindof suck in PVP, because their T1 set is pretty garbage, but all of their counter classes (mage, warlock, hunter) have pretty good T1 sets. That said, 1-2 warriors will still be desired in RBGs at least until naxx is out, and the 1.12.1 itemization helps warriors A LOT between BWL-Naxx if they get rank 13/14.
Hunter - the best class for leveling, and for solo (single target) grinding. It can also hold it's own in PvP. However in raiding, post MC, it scales very poorly, and you will never be anywhere near the top of DPS charts in raids. So if that's important for you, do not chose hunter.
IMO hunter is THE BEST open world ganker, if that's your thing. The only classes that can really fight them are like: druid(if EXTREMELY geared as feral/boomkin, so, basically these don't exist), warlocks, and Spriests(also don't really exist), and honestly they can just choose not to fight warlocks and there's nothing warlocks can do about it because they're so slow.
Hunters are naturally pretty decent DPS in MC-tier gear, but 1.12.1 itemization (comparatively) hurts them a little bit, especially with Lupos not doing shadow damage. THAT SAID, in BWL Hunters will be able to do competitive DPS with at least the casters if they get rank 13. The PVP sets of hunters is basically equivalent of T3 without the set bonuses. They will def go back to sucking in either AQ or Naxx, tho.
Warlock - best all around class choice. Is good at everything, PvP, raiding, dungeons,... and if Blizzard ever decides to expand classic with retail TBC again, warlock is the TOP class in TBC without any competition.
I agree with Warlock being one of the top 3 classes, but I would add: WotF REEEEEEEEEEEEE, and a big downside to warlock PVE is that they have the most/expensive consumables of any class by a far margin. Additionally, they kinda suck without gear. Their T1 is really good for PVP, though, and even if you're casual T2/2.5 is really good PVP gear, and they get a lot of "free" gear that's super good for PVP, and PVP. Like, the fricken AQ20 ring is their third BiS, and the ZG cloak is basically BiS. it's super easy to gear a warlock late game.