A few days ago, on Google+, Dominic Crouzet posted on what he felt was missing from Fantastic Heroes & Witchery - and explanation of what Thief skills actually are:
There is something I regret to not have thought of when I wrote FH&W. Recently I did read a document over at DF, where the author explains that (in his OD&D game) everybody can hide, but only thieves can hide in shadows; everybody can steal purses, but only thieves won't be noticed; etc. I think that would have been a nice addition to the FH&W thief:
Thieves have skills that all other characters can also attempt, whether trained or not, even though thieves get more bonus to their skill checks. However, the thief class can do things with each of these skills that no other class can do. Specifically hide in mere shadows (where others can only hide behind something); steal without being noticed by nearby witnesses; climb very smooth walls; etc. That would give something special to the thief class.
I think that this is very important. Obviously, it acts as a corrective to the 'underpowered and incompetent' interpretation of the Thief class without changing anything but the perspective of players and referees - those laughably low chances of success are in fact the chances of the Thief doing something remarkable. This is because classed and levelled characters are exceptional people. Not all thieves are Thieves. The proliferation of classed and levelled NPCs in D&D material was one of the big wrong turns, looking back at published D&D material. Even in the marvellous Night's Dark Terror, Threshold is home to an unnamed 7th Level Thief who, improbably, seems to make his or her living picking pockets. By understanding success at Thief skills as the achievement of something remarkable, the petty pickpockets can be 0-level humans performing mundane, rather than exceptional, acts, Any PC should have a crack at performing mundane acts, depending on circumstance. But only classed characters get to use class abilities in order to achieve the exceptional. And by considering Thief skills in this way, we effectively grant high-level Thieves the kind of superhuman abilities gained by the other classes at high level - there is no need to detail new abilities as promised by Cook and Marsh.
But, of course, not all thieves are Thieves, not all 'fighting men' are Fighters, not all priests are Clerics, and... not all students of magic are Magic Users? Yes, possibly even the last case should be true. There should be 0 level scholars, cunning men and wise women who can work some petty magical effects, at great expense, effort or sacrifice, but only classed and levelled Magic Users can work magic with true power. And once the players and referee start thinking about classed and levelled characters as exceptional within the game world, playing at low levels takes on a different flavour.