Due to various content and publishing reasons, Chinese players have only just recently been given access to the wonderful Burning Crusade expansion, and true to form, they've already conquered the record for quickest character leveled to 70.
Only 23 hours after starting his journey into Outlands at 11:20am on September 6th, troll hunter SilverDragon managed to ding 70 the next day at 10:48am. The previous record was held by the French player Gullerbone, who achieved the task in 24 hours with the help of his guild, who would assist in killing mobs he tagged in order to maximize his experience gain. Whether or not SilverDragon used the same tactics is hard to say, but from the looks of some of the screenshots on this site it looks like it might've been a group effort.
