Nah they would have teams working on Diablo separately, there's also Starcraft 2 as well don't forget hoping that will come out by christmas! - nothing to do with wow being the reason. They have made WoW so easy to play now for new players that with BOA gear (Bind on account) you can get from lvl 1 to 80 in 2 weeks (not gametime) or a a bit longer if you play casually its so fast that its turning hardcore players to leave the game - and not to mention end game instances are a piece of cake. Then they copied Warhammers style by allowing the player to level when in Battlegrounds pvp now so if your not into questing which i really haven't got time to run around and grab 60 kodo skins etc., then lvl up in pvp - although lvling up in pvp style can cause you to go into fits of rage and become a stalker in the game searching out that punk that ganked you 4 times in a row then laughed at you and teabagged your character just to rub it in!
One night in Warsong pvp i had the whole alliance side chasing me around the map bent on ending my life i so wish i had fraps running at the time as it was damn funny to see all these players after me - how did it start a hateful rage? I laughed at one of the top players i just ganked (Konradcurze is ROTFL at you) While i was running from them in utter terror
our team manage to capture the flag 3 times and win.
Anyways back to the topic
