Thanks, everyone, for your comments on the Monk class in D&D. One thing that stands out is a sharp divide between those who like the flavor and implied setting of the monk and those who don't. Thematically, the Monk is (a) "Eastern", not "Western"; and, (b) "wuxia", not "picaresque". The East/West split isn't as big -- a monk character could be a foreigner, after all -- but it does ignore the fact that the West had monks, too: they just had a different focus. The wuxia vs. picaresque split is much bigger, because it not only makes the monk stick out if you intend to play all-picaresque, but the class is also poorly designed if you plan to play all-wuxia: the monk is very weak at low levels, much more so than the thief, and that offends people who would otherwise love playing in a wuxia style.
Mechanically, as Richard noted, the Monk is a grab bag. It's always represented to me the problem with most new classes: class design over the years drifted more and more towards "let's see how much stuff we can pack into this class". It's not that monks shouldn't have thief abilities, or unarmed combat abilities, or mystical powers, but that giving them all these things together makes for a cluttered class.
What I'm thinking is that several of the monk's special abilities should be switched out of the class into separate abilities acquired through adventure. "I'm going to seek out the mysterious Grand Master of Flowers and learn the art of the Quivering Palm he's reputed to know." In a sense, the abilities become "intangible equipment", and the desire to have that ability generates adventure, rather than merely being a by-product of it.
I'll have more thoughts on what I think the base class should be like later.