I only got one day of beta, played thief to 13. As a long time rogue/stealth (ok, griefer), I was a bit disappointed there was no permanent stealth. This class is NOT a rogue. However, I think it will be a very powerful pvp class.
Thief has medium armour, so not so squishy, but no tank.
I think the key tactic for Thief is the jump. With a melee weapons rig, you can jump in from range, burst damage, back stab where the computer moves you to the targets back, smack, smack, smack, then jump back to your starting point. So, it looks to me like its a leap, stun, burst, leap away tactic.
The twin pistols are powerful for ranged.
I didn't like pistol / knife, as it was a bit of a hybrid. I prefered pure melee, or pure ranged, with melee jumping a lot.
Thief seems to have escape tricks, which should make him hard to kill in the hands of an expert.
Also, consider 2-3 seconds of stealth, combined with jump away. So you could leap in from an unseen location, kill a target among a group before the others know what you're doing, stealth and jump, so they can't see where you have gone, then run!
Only got to 13, but this is the way it was building for me. Lots of potential. 2-3 Thieves working together with jump/burst/stealth/jump tactics might be quite a problem. Time will tell.