A few observations resulting from a lot of experience with retail, classic, and the unspeakable.
I hope I can help people decide what to play and how to play in the coming expansion.
PVE will be easy. In my opinion - easier than non wordbuffed Naxxramas (excluding Sunwell). There will be outliers like entering Hellfire Citadel heroics on your blue/green geared tank and getting pummeled into the ground by muscular orcs, but those will only last a few weeks max, then you will have farmed/cafted your gear and become familiar with what to pull and how.
It will be possible to clear all content with multiple suboptimal specs like shadow priests, but most guild will continue to metaslave and will try to produce "optimal" comps. This means some popular classes will struggle for raidspots (sup fury warrs).
"Pre nerf content" - one of the selling points of how TBC is going to be "harder" - don't believe it, it won't matter. First of all - only a handful of bosses were meaningfully changed and even then it was rather a different version of mechanics (like Solarian pre-nerf bouncing debuff or Mother Shaz prismatic auras) than a big difference in difficulty. Asmongold just made a video talking about big scary mind control on Vashj tank. Guess what - just bring an offtank to taunt the boss
Or bring five, because the DPS check will be irrelevant.
The only thing that will really matter are 360 degree (chain) cleaves that were a thing before 2.1. They are deadly to melee and require knowledge of the mobs. Sometimes melee just don't go in on packs at all due to those. Unless they are fury in plate
Spec rating - in order of best to worst (generalized, can't really compare tank healer to a raid healer).
Tanks:
1.
Paladin - there will be at least one in every raid, and if bosses hit for what they did in retail TBC, they might be able to maintank everything up to Sunwell with good healers and/or externals. As a bonus they can solo farm places like Strat/Scholo for 200-300g/hr (or more - depending on the realmeconomy). Best 5man tanks by MILES.
2.
Warrior - awesome single target just like in classic, but severly limited in AOE tanking. High skillcap unlike "drop conc" paladin (a meme... or is it?)
3.
Druid - quite decent with 2.1 itemization, threat machine for Sunwell bosses, most likely will be required by many guilds at that point.
DPS:
1.
Warlocks - very good single target, awesome AOE, but no cooldowns. The faster bosses die, the worse locks are. They will be present in speedrun meta, and also in more casual guilds. Don't worry about there being a lot of them - it will be easy to find guild spots.
2.
Hunters - awesome single target, crappy AOE, great cooldowns. They will not be in speedrun meta, but standard guilds are very likely to have a "hunter group" (at least a feral, maybe even enha too). Many spots, easy to get into guilds. Considerably higher skill ceiling than Warlock due to pet control and punishing pet mechanics. The best dps class in Sunwell.
3.
Mages - mediocre as frost or fire in T4, extremely good as Arcane in T5 with proper gear. Spots limited dur to requiring innervates to perform best. Better aoe than locks and whats extremely important - a LOT less threat generated. Fade off hard in T6 and beyond.
4.
Warriors/Ret Paladins/Rogues - with proper group/debuff setup fury can be very competitive with 2.1 itemization and BIS on cleave fights. Problem is there will usually be 1 spot for them in a normal guild. Arms warr will likely be brought for debuff, just like one ret pala. One rogue spot might or might not get filled to gear a guy for Sunwell progression. No reason to bring more, they will struggle before T6 gear.
5.
Boomkins - 1 spot in warlock group. Highest DPS on most fights among hybrid casters if played well.
6.
Ele Shammies - 1 spot in warlock group.
7.
Enchancement Shammies - might bring one to buff melee group, not guaranteed. Brings lust, thats a big plus.
8.
Shadow Priests - 1 spot in healer group if guild struggles on mana for some reason and to buff warlocks. Unlikely to be really needed due to fights being short and healer mana not being relevant unless it comes down to dropping to smth like 2-3 healer comps. 1 spot max, crap dps.
Healers:
1.
Shaman - you bring lust and that makes you BIS. Multiple spots
2.
Resto Druid - best raidhealer. One spot.
3.
Priest - one spot.
4.
Pala - tank healer, one spot. Might have to compete with shaman, depending on how the guild values blessing vs additional lust. High output will not be needed in the unbuffed content state.
There are big differences in pre-bis gearing of some classes, for example Warlocks are going to be very expensive while hunters are pretty cheap. Farming primals will be an absolute cancer on populated realms even with layering. Think Ungoro mafia but almost everywhere.
Feel free to ask more specific questions.