http://us.media.blizzard.com/wow/promotion/wap/en-us.htmlSo I guess this is how Blizzard is planning on keeping people in WoW after the release of Star Wars: The Old Republic and Guild Wars 2.
If you agree to pay for 12 months of WoW (not all at once, you just lose the ability to cancel your account), you get:
Diablo III full game for free (probably no discs, just added to your B.net account and you DL from there)
Tyrael's Charger mount for all existing and future created characters. Works on land and in the air. Looks like a Paladin's charger, but with Tyrael-esque glowy wings.
Guaranteed beta entry for The Mists of Pandaria
Diablo III is $60, and the mount is something they would normally charge $25 for in their store (there are multiple similar mounts for that price already). If you're planning on playing WoW for a while longer, this deal is a no-brainer.
