Like the magic-user, the thief is a very "weak" class. Only leather armor, d4 hit points, limited weapon selection, etc all make the thief sneaky and combat averse. Menzter gives this advice directly: "when an encounter occurs, stay out of the way." The thief, then, falls back on her Special Abilities (interestingly, this is always capitalized in the text)(1). But these are horrible, at least in terms of chance of success: 10% chance to find or remove traps (separate abilities here, though the percentages are the same), 10% chance to hide, 20% chance to pick pockets. . . The thief does have an 87% chance to climb walls, though. If you are going to fail 9 times out of 10 in performing your special abilities, you not only have to have some restraint and patience, but you also have to be creative in ways your character can contribute to the party's success when your abilities fail (as they will most of the time). The XP curve is a lot nicer for thieves, however. The thief is at 3rd level well before anyone else. The thief player, then, needs to be able to figure out how to use her abilities as best she can, as well as contribute and not get smashed by ogres until things get better for her, which it will soon.
I think a lot of the player skill, though, will need to come in interpersonal skill with the players and DM. The character is a thief, after all. She steals stuff, though only "rarely" from other party members. This can take some delicate balancing, depending on party composition. I am beginning to think that the party is not a bunch of "murderous hobos" by default in Menzter, so the thief occupies a unique sort of grey moral area. This is certainly true when the Guild is factored in. The class description states that all thieves are part of a Guild, all thieves learn "The Arts" from a Guild teacher, and all towns have a Guild Hall! This is a fascinating bit of world-setting, with potentially significant campaign implications! Leaving that aside for the moment, it also potentially places more demands on the thief's player in terms of role playing and sorting out her role in the party without being an ass to the other players by stealing their stuff. (2)
There also has to be good player-DM communication for the thief. The default is that the DM rolls for all the thief's
I hope I am not sounding critical of all this. I personally prefer some player/DM back and forth over all of this than six pages of modifiers. The former is a major tenet of Old School gaming, I think. I just think it's interesting how much the way the class is set up implies that the player of the class will be able to do certain things around the table.
(1) I find it interesting that they are called "Special Abilities" and not "Skills". That certainly implies that they are unique to the class, much like Turning Undead is to clerics, and not something that fighters should or could be doing.
(2) The Easley illustration of the thief certainly does not help the class's trustworthiness. The example is not a fresh-faced young adventurer, but a middle-aged man with a cowl and a sneer, who is looking back over his shoulder, presumably to both check he's not being followed and to gloat at the dead body of his fighter "friend" who is now busy getting eaten by fire beetles while the thief absconds with a sack full of silver and electrum pieces.