I've thought a lot about this. I even just deleted a big post outlining the few things I dislike. I think, in the end, the lens of Retail is what will paint my impressions of those systems I don't like, but if I view those systems through the lens of Classic, an MMO-RPG, any system I may not like actually is a system that contributes to the RPG element of the game.
Weapon skills, for example - does it suck when you get to 51, do AV, get Ice-Barbed Spear, and have your Polearms at level 20, when they need to be closer to 300 (that was the 60 cap, right?) Sure. There is tedium in levelling a new weapon type. You know what else there is? Immersion. This is someone who is not skilled at that weapon type and must practice the proverbial 10,000 hours to get good at use. I love that! Jeez, I levelled my unarmed to cap in Wrath to get the achievement! It may be nice to pick up a weapon and who cares, you're just great at it, but the RPG aspect for me far outweighs the hurdle, and in fact I'm looking forward to it.
Corpse runs - another tedious thing, but it makes more RPG sense that your ghost would spawn at a spirit healer in a graveyard rather than magically appearing, alive and well, at the beginning of the instance.
Anything I may not like for one reason or another, if I look at it through an RPG lens immediately becomes something I'm not only ok with, but I'll be happy that it exists the way it does. This is a different game from Retail, and always will be. I have no doubt that I'll complain about this or that once I'm actually there, but anytime I do, I'll take a step back to look at what's bugging me, and I'll know that even though Retail may have addressed and changed that specific issue in the modern version of the game, the magic of what Classic is will more than make up for the inconveniences.
I'm just kinda too optimistic about Classic