I would agree that a thief will find it very tricky to fight even a sub-standard equipped warrior with poison who knows what they're doing even a little bit. However, patience will bag you that opportunity to bag them, at times. Thieves aren't warriors, and they certaintly wouldn't be expected to fight toe-to-toe with them. However, where I agree, is that it can be tremendously tricky to fight them in a hit-and-run style also, because a staffing warrior can win a fight by using bash alone against a thief, especially if they're enlarged.
I have a couple of ideas. I'm not saying thieves need changing at all, because with patience and preparation they can take down a warrior providing they aren't decked. Which as a thief, you'd be working on making them not decked anyway. 
First idea: A defense, similar to dodge, perhaps acrobatics or something, that grants a small chance of evading bash/bodyslam. It would have to be quite a small chance of evading, so that it's not impossible to laglock a thief, but then the problem there, is "is it actually worth it"? It usually doesn't take more than a few bashes, especially enlarged, to have a thief dead unless the thief is decked and the warrior is equipped to poor standards.
Second idea: Same acrobatics style skill, except this one isn't passive. Basically, in combat, you utilize the skill and then for one/two ticks, you have the acrobatics effect. Which basically gives a very good chance of dodging ONE bash/bodyslam, before it must be used again. The skill has a one-round lag effect when used so that if used, he has enough time to run/do something then run if the bash/bodyslam fails, with it's three rounds of lag. The effect will be removed if either the thief or their opponent flees from combat, and there would also be some form of timer before the thief can use the skill again.
Again, just a couple of ideas (with flaws) to bring up, not saying thieves need anything or not.
As for weaponry, I think being able to backstab would help, as swords are more defensive then daggers/spears, but there are some pretty unwieldy swords out there that IMO you wouldn't even dream of stabbing with. 
Dey