Didn't play a lot of thieves, eh?
Thief skills advanced rather slowly and started abysmally low. And nearly every roll was modified by some condition. Some were mentioned in the skill itself, some in the equipment section, and some hidden in the DMG. But if the DM wanted to, there was always a way to penalize thief skills.
In theory, a high level thief was great at thievery. In practice, any challenge worthy of your level was most likely penalized. So it was great if you liked picking on low power, low value targets. But to be honest, I'd much rather have expertise and reliable talent to show off my mastery of thief skills.