It is often claimed that the Thief is the hardest class to play.  Certainly the armor restrictions will hamper combat ability, but no more so than any other of the thief-type classes.  The Lockpicking and Backstabbing in the Major skills will make it difficult to reach the highest levels, but are not an overwhelming obstacle.  The high Short Blade skill, coupled with the Ebony Dagger and a reasonably good Critical Strike skill make combat more survivable.  At higher skill levels the Thief is capable of inflicting serious damage through Backstabbing and the high SPD and Running skill means he can hit and git with good chances of surviving to do it again.  The Thief should join a temple early in the game in order to gain access to potions and items.