They are making several changes to Classic from Vanilla. Some immediately obvious, some not so much.
I won't spoil the cosmetic/quality of life changes, but I will say this (don't quote me lol):
I agree with Motorbreath when he says 1-60 will NOT be a fast process, unless you are one hardcore gamer.
If I remember correctly they are blanket increasing monsters armor (and hp?), some mobs have their resistances changed, and some mobs have new abilities (Example: Fire Roc in Tanaris now has a fireball cast, no longer melee only, sorry mages )
Resulting in a ~20-30% increase in time required to hit 60.
Should be fun
