So, let's just assume for a moment that you had the best, 25 man Heroic gear, fully enchanted and socketed, you've completed every quest, done every raid, gotten all the mounts, etc.
What's your favorite thing to do after that? Wintergrasp/Tol Barad? RP mania? Running around on Timeless Isle?
[Just a note, when I meant by beating WoW, I meant basically completing all of the main quest and getting best gear possible).