I am a veteran WoW player, and I played retail since first vanilla release until the Lich King (around 5 years) at which point i quited retail because I didn't like the path at which wow was going.
After that I spend additional years playing on private vanilla wow servers, for nostalgia, and for social aspect.
I honestly don't know if I will venture into wow classic... 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.
(Many casual players don't know half these things, even if game was out for so many years, for example... most melee players still think that you have to stand behind raid boss only because so you could use "attacks that only work from the back", and so that boss wouldn't cleave you to death, while being completely oblivious to the fact that the most important reason to stand behind the boss is so he wouldn't parry your attacks - he can't parry from behind - so that resulting speed increases he gains wouldn't kill the tank).
Many of those little things are getting tweaked for new classic, and classic will mechanically be different experience than retail vanilla was in old days.
Many of those changes are still kept hidden by Blizzard, and will not be revealed at all (unless discovered by players), so we can't say for certain how the things in new classic will look like yet.
Content wise, it will remain the same, and since I played this content for years, again and again, this aspect is not appealing to me anymore... perhaps I would join during first month or two, for low level world PvP while zones will be filled with people rushing back to wow for nostalgia, and when I reach 60 I might retire from there, as I raided everything to the death already and from that point on, there is nothing new for me to see, content wise.
Class advices for those of you that wish to venture there:
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.
Mage - good all around character, but bit boring to play in raiding, basically you will only spam frost bolt button from MC until AQ40... however, its a great class for AoE grinding, and preety fun to play in PvP. Future wise, in TBC it becomes not so great class to play.
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.
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.
Rogue - leveling efficiency greatly depends on keeping your gear up to date, as without that it's a pain to level a rogue. At max level rogues are great in raids, and can also be fun in PvP (for people that have patience to stealth around looking for opportune targets).
Priest - the best healer in vanilla, and is also good character to play in PvP. Priest is also a good choice, but consider that in raids you will have to be a healer, and changing specs between healing and DPS/PvP can get pretty expensive if you wish to do both regularly.
Shaman - you will be asked to be a healer in raids, DPS shamans aren't that good in vanilla. Can be great utility class for PvP, if you want to be a PvP healer, Shaman is a great choice. Will require a lots of micro management though.
Druid - you will be asked to be a healer in raids, tanking druids are less valued in vanilla due to some mechanical disadvantages over warriors. Druid is a hybrid class, which is decent for solo play, but not so good for raiding where you have to specialize in one role, druids are taken to raids because of buff they cast, combat ress and innervate, but you will not heal better then priests do.
Paladin - you will be asked to be a healer in raids, due to some mechanical disadvantages over warriors in vanilla. Paladin is pretty decent class to play if you don't mind being a healer, its invaluable in raids due to great auras it offers to players in your raid. Do not expect to ever be a tank or DPS in raids, and even if raids let you, you will perform mediocre in those roles at best.