Bans are necessary to keep the online gaming space as jerk-free as possible, but some players think that Blizzard might not be paying enough attention to who they're banning. Several complaints have surfaced from World of Warcraft subscribers who were ticked that they were banned for no reason, and with no warning.
Mistakes are going to happen, of course, and with roughly half the planet's population playing WoW, it seems reasonable to give Blizzard a bit of slack if they mistake some aggressive advancement for power leveling. The problem, though, is that apparently these folks who've been wronged have had very little luck actually getting in touch with Blizzard to plead their case. Also, Blizzard has taken to banning known associates of guilty parties; so if someone in your guild is an asshat gold farmer, you may as well start packing for your trip to Banville, too.
Blizzard, for their part, is standing behind their current banning practices.
