It needs to be said that TBC needs changes to avoid serious pitfalls.
If they stick to maximum authenticity (#nochanges), then a lot of things will fall apart in TBC.
Using 2.4 Talents alongside a 2.4 state of raids / dungeons will make most content extremely easy, not to mention that Attunements were completely neutered before 2.4 already.
If they also stick to the original release schedule, you'll have people running around with Mount Hyjal gear in week two.
Which will then obviously impact Arena by an insane degree if PvP'er only have S1 gear available while PvE'er use T6 equivalent gear.
Then there is the whole faction balance issue, Horde was heavily favored especially in PvE in TBC, the Seal of Blood issue is probably the biggest culprit here.
If factions are too imbalanced, then you'll probably repeat the issue that AV has right now, Alliance has fast queues but loses every game, Horde has hour long queues but wins every game.
Then there is the thing about outdoor world, some Mega Server would need a ton of layers because Outland is much smaller than Kalimdor + EK, imagine the people that hang out in Org / SW are now suddenly thrown into one city, Shattrath.
Too many layers obviously has issues with how the older iterations of the game are supposed to not have this sort of thing.
However, i do not believe that TBC will beat Classic in terms of hype, TBC is great, but Classic is the original game, TBC will likely have a higher retention rate than Classic, but the initial hype of Classic, of it being completely fresh with nothing else on top / below it, will be difficult to surpass.