trust me when I say what u get in private servers are 10% or less of what paying subscribers are experiencing.

Zero bugs (It's a whole different world, seriously) in talents, quests, instances, also in scripted events like battleground weekends, fishing tournaments, arena pvp, working different types of battlegrounds (point capture, capture the flag), faction rewards, profession perks (i.e, discovery for alchemy), quest events, major one-time events (gates of ahn-qiraj), holiday events (Lunar festival, Hallow's End),

The sort of folks you meet are much more diverse, racially and in form of maturity (although u will meet your share of scrubs). I just started and some friendships I made have potential to go far. You get first dibs on upcoming changes in wow as well, instead of having to wait for a server update (and probably incomplete or flawed) for the private ones.