With 10% bonus movespeed, 3 ways to go into stealth with two of them healing me for a total of 3 heals (I set my underwater heal to a different skill so when I jump into water it is off cooldown), an immobilization and a cripple for 50% I really don't have a problem getting away from anyone... No to mention worst case scenario you roll of a cliff. I got a lot of kills by rolling off a cliff, stealthing at the bottom. Safe to say it is possible to get away from anyone if you use the landscape properly.You clearly haven't come across a decent staff mesmer or longbow ranger if you haven't gotten owned by range. A ranged weapon set is critical for all classes but the thief. (especially warrior/guardian/ranger) Even then a pistol offhand is pretty beneficial for the extra teleport.
I think the problem a lot of people are having is they want to initiate right away at which point they will get kited. You need to bate their skills out that get them away/knock you away and take a bit of damage and then jump in. Melee does easily twice as much damage as range so if you take a bunch of damage it is still worth it to get a good initiation. Especially as a thief where you can hit haste stealth and then crit them in the back for 6k followed by your epic skill which will reflect anything they try and hit you with, while damaging and slowing them.
The more I talk the more I think maybe the thief needs balancing, but then I remember what guardians do to me. Thieves can't do much against guardians but they can crush those squishy casters and rangers. Thieves counter range, guardians and warriors counter thieves. Engineers built a certain way are a big pain in the ass too but they can't kill you, you just can't kill them either.
My set up is dual daggers and pistol-dagger (pistol being main hand). Always gotta have that slow and and stealth in 4 and 5. Skill three helps get out of bad fights too, use that and stealth and by the time they see you, you are long gone.
I actually plan on playing a staff mesmer on release.