Wow...you guys are mean lol
Anyhow, I leveled as Concealment and never had a problem...could even solo Heroic 2+ quests so long as they didn't stack the deck and I only had to kill a few elites+grab an item and get out...
So, I'd say Concealment if you like living on the edge (will probably die a few times until you figure out how to best utilize the mechanics)
Or Medic for a more stable, albeit slower, approach to leveling...won't die nearly as much, but won't put out nearly as much damage either...
The nice thing is that no matter which spec you choose, you get stealth which is an immense help all around....and Concealment utilizes stealth to it's maximum effect