Gameplay changes would do it. Especially any change to scaling, mobs should be just as they were when the game was live, right down to the elite mobs in obscure areas like that troll fortress in the Hinterlands. It all needs to be the same gameplay wise.
Im expecting graphics changes, BNET integration, and i dont really like it, but expanded add ons that were not around when the game was live. Directly altering the gameplay i will bail though, its not a classic server in that case.