Classic server won't keep that interest though... The problem is it NEEDS to be progression though if you want to keep people interested, no one actually wants to play for 6months or whatever completely cap out their character and then have nothing to do.
15 man? F that we would take 40 in and blitz the hell out of it to attune people.
I don't even remember a naxx attunement so I don't think it was a big deal
Best one I just remembered was taking a shit ton of people into Stratholme etc. and because you was in a raid you couldn't complete the dungeon quests so you dropped raid, clicked on the item, got an invite back. I always thought that it was odd they had raid quests that you couldn't complete in a raid only to find out that you shouldn't be in a raid group doing this content.
Diablo 4 is pretty much guaranteed to be an MMO at this point, from their hiring posts.
Probably comes out after the "final" WoW expansion, whenever we beat the void lords or whatever. So that players can transition from WoW to World of Diablo4craft.