Evil is Back - Diablo III Launches
As with any popular launch day online title, Diablo III has certainly had its share of launch day problems. Blizzard made Diablo III require you to be logged in via the Internet to play with no option for offline play, so this compounded the problem of congestion. This was accompanied by a number of other issues that popped up to hinder some players trying to get started even if they did manage to login that included a grayed out character screen, random disconnects and a series of different errors that often landed them back in queue.
The Battle.net website was taken offline temporarily and more servers were added to accommodate the heavy load. With the worst part of the launch bottlenecking hopefully behind us, it should be a bit easier to join the fight against the demons of the Burning Hells from here on.
Source: Diablo III Launch Announcement