From what I've gathered we know very little of them, but I'd like to know if any of these were answered somewhere:
What version of Vanilla are we talking about here?
Will talent trees be the same as they were at launch? Will balance druids have oomkin form or is the ultimate talent going to be hurricane?
Will there be diminishing returns or are warlocks able to fear you untill the very end of days? Will all the class changes come in waves like they did in vanilla or what?
Will the honor system be in place? Is it going to be like it was for the most part of the xpack (as in the worst and unhealthy system imaginable) or will it be like it was during the tbc prepatch? Will you have to walk to the battleground entrances to queue or will there be battlemasters? Maybe there is no honor system or battlegrounds?
Will all the raids be available straight away or will there be tiers? If they are all available straight off the bat, then there's really no use going for MC or or tiers like that. All the encounters are too familiar and simple to modern players.