With 6 million subscribers, each of whom pays $15 a month, Blizzard Entertainment's online game "World of Warcraft" has become a billion-dollar enterprise.

Now comes the hard part: Making sure WoW is always up and running. Some players are angered by ongoing server problems that have led the game to crash without warning while they were playing. Complaints have also surfaced about long lag times and frustrating waits to even play.