I played pretty hardcore in vanilla, semi hardcore in TBC, leveled in wotlk and then quit. I got the classic bug (and enough time to play) ~3 years ago. I played on Nost, raided on Rebirth, raided on Kronos, raided on Elysium/Light's Hope, leveled in Northdale (but had a change in jobs and no more time to play).

I think the biggest difference/challenge for classic is going to be how much min/max info there is and how fast people tear through content (and how that will ruin the "nostalgia" feel after a while). Even the "nostalgia" people that play classic will eventually get 60 and want to keep going. Specs/equipment/consumables/strategies are so specific/efficient now, that you are pigeon holed into playing a cookie cutter character or are considered someone that is hurting the group and slowing progress. Yes a spriest/feral druid/enhance shaman can raid, but any hardcore/semi hardcore group won't want them as a rogue/warr will bring so much more dps.

I think the last launch I was on (Northdale) had its first 60 in 3 days and all content cleared in <2 weeks. It was like that for every patch....there are no more "prep" raids where you learn an encounter, develop a strat, gear up, etc. People will be prepping for BWL/AQ/Naxx for months and will clear them the day they are released (maybe not Naxx). I was a good/great player in retail, on private servers I was just average because to be great you have to farm a never ending list of consumables (top dps will always flask, etc.). Eventually all you do is farm consumables and prep for raids.

The economy will also be interesting. People know exactly what items are needed and when (for example Elemental Earth for AQ) and can really play the AH and hoard items, then sell when the market peaks. Mages will hyper inflate everything once DM comes out, and then you will also have "mafias" controlling things like the black lotus and devilsaur leather market. I never really remember any of this in vanilla, if people knew about that stuff it was kept secret.

I got my nostalgia fix and did enjoy playing a lot. But the nostalgia quickly faded at the end game, and I burned out enough that I'm not even going to play classic.