well to be fair there is a wall, i mean if your in the top guilds for example and your level 70 knocking down all the bosses, after youve taken Illidan and everyones got their gear (or more selfishly youve got your gear) you dont really need to play much anymore, same with PVP if youve done the first part in PVE and you like PVP you can try hit high warlord or something or champion Arena team ,but the game is not truly endless, i mean sure you can still run around after youve done everything but the content is not infnite..ofcourse Blizz may bring out a new xpack (ROTLK) so youll have more to do when xpack comes out