Wowhead is putting out a series of returning player guides – even if not a returning player the changes are interesting to see
Vanilla
http://www.wowhead.com/returning-player-guide-changes-classic-vanilla
Burning Crusade
http://www.wowhead.com/returning-player-guide-changes-burning-crusade
Wrath of the Lich King
http://www.wowhead.com/returning-player-guide-changes-wrath-of-the-lich-king
Cataclysm
http://www.wowhead.com/returning-player-guide-changes-cataclysm
Mists of Pandaria
http://www.wowhead.com/returning-player-guide-changes-mists-of-pandaria
Warlords of Draenor
http://www.wowhead.com/returning-player-guide-changes-warlords-of-draenor
None the Wiser Guild 