Wotlk IS and HAS BEEN the most popular expansion of World of warcraft in the private server scene.
It is also the most engaging and replayable expansion since it's still the old wow that we loved but more polished, yet not the new kid lfr trash that became in cata and everything onwards with the old world revamp
Not so hardcore as TBC and not so easy as cata to gear up, will be cool to gear and also let people have some alts to have fun...
Also arena is fun as hell and more balanced than probably any other version of wow but 5.4 patch...
To be honest and actually the only version of the game to bargain on at this point