Classic was great for it's time.
There is many things wrong with it (many things right) and I don't want to see many changes.
But let's take a sensible approach to it.
Questions
-Do you really want to drink after buffing (when in sufficient gear, I still think classes should have the items in their bag)?
-Do you really want crap UI addons? (by UI addons I mean graphic only, but don't kid yourself, all vanilla bosses are simple compared to today and we know all tactics. Addons like quest helper exist on private servers, so that means they will exist on the classic, what is missing is up to date graphic UI addons).
-Raid drops, will you be happy raiding MC and bosses only dropping 2 items, sometimes for a Shaman/Paladin that you can't use? This means when BWL and other raids come out, it will still be a requirement to raid older tiers (which isn't really a bad thing).
-As a warlock will you be happy grinding kills before every the raid, to hand out HSs and trade them individually?
-Mages, will you be happy to trade your entire raid individually to give them water?
-No dual spec? I remember having tanks do nothing but dps (well you can call it dps I guess) because a fight didn't require a certain amount of tanks. People didn't like respeccing due to the cost back in the day, but private servers have shown that it is easy to make gold when you know what to do.
-I'm unsure about the length of Paladin buffs (I played Horde, but in TBC I always enjoyed annoying my Paladins for the Salvation buff to be renewed via macros).
-Windfury totem should definitely remain
I'm sure there are many more minor things that can be improved.
I don't want any class balancing, changes to the talent trees, changes to the PVP system, changes to levelling speed, changes to gold income, I just think there is small issues which are more annoying than anything and will not change your experience at all, unless you consider not drinking after buffing game changing.
I'll play regardless, I just think there could be minor improvements made which won't change the experience for new people.