The thing about MMO's is they're constantly updated, bugs are fixed and new features are added. That's the benefit of paying a monthly subscription. There's also the GM's, account support etc which is always far, far more reliable, faster and of better quality with a paid MMO and a F2P MMO.
WoW itself I've tried and I'm not a fan, but when you consider I play City of Heroes every night, often for 3-4 hours and today i've played for about 5 hours then the cost per hour is minimal, often in the pence.