Another reason for the slow release schedule is the haemorrhaging of staff from Blizzard North, followed by its subsequent closure and cancellation of the work it did on Diablo 3- http://en.wikipedia.org/wiki/Blizzard_North
Without that, it's likely that Diablo 3 would have been released already.
I really hope Starcraft 2 and Diablo 3 live up to expectations. The RTS and RPG worlds have moved on a lot since their prequels were released and people will be looking for a lot more than a clickfest and a traditional sci-fi RTS. Hopefully, the extended development time and the fact that WoW's enormous success means they don't have to release anything until they're absolutely happy with it, will mean they're excellent. I feel they do need to do something different for subsequent games though. Warcraft 4, Diablo 4 and Starcraft 3 in 2012 isn't exactly something to get excited over.
