Nymh of the french server Drek'thar was the first character to level 80! She plays as an Warlock. She used to be the guild leader of Harmonie, a french top PvE guild who was among the first to kill Kil'jaeden. She reached the max level 27 hours after the european servers went online, that's pretty fast on leveling. Because that she decided to level from the beta servers. And it took about 3 hours to level from 78 to 79 and considering that it was one of the longest level, it seemed good enough to use this strategy to powerlevel at the release.

The biggest problem was to find a spot with enough monsters to grind without too much downtime. Eventually found the crypt in Drakil'ji Ruins (Grizzly Hills), that's one of the only place in Wrath of the Lich King where monsters repop in about 30 seconds, mobs here are also neutral and it makes grinding a little safer.
She had to optimize my downtimes and decided to level with a healer out of party, Pavelomm was nice enough to spend the last 27 hours healing without taking any XP and he obviously played a huge role in the process.

When she reached level 76 she realized that her leveling speed was slightly lower than what I experienced in beta, I decided to change the template from the original post and switched to something with 2 points in Shadow Mastery to make my DoTs stronger and the leveling a bit easier. (It usually takes 2 DoTs to kill a monster)