Monk was a true genius. A true original; and like many geniuses, he was eccentric and this was reflected in his music. Underneath the eccentricism was a bipolar disorder that would eventually lead to his withdrawal from performance altogether. The genius of Monk's playing was in his ability to somehow make so many of the usual "musical taboos" swing and be musical. Like Detlof said, he often hammered chords on the piano instead of aiming for tasteful and well balanced voicings. The voicings he used were often very dissonant and he loved to lace his improvised melodic lines with dissonant major and minor seconds. Rhythmically, he didn't play with the usual smooth and suave sense of swing of many players and instead played with a rather "square", angular and fragmented rhythmic sense with accents in unexpected and unusual places in a phrase. Yet, it all swung like mad somehow.
The subject of his technical ability was equally eccentric. He studied classical piano and could play Liszt and Chopin so one can assume that he had some real chops in spite of whatever physical limitations he may have had. He loved Stravinsky and Bartok and I think this influence was reflected in his playing. However, his musical vision was not about technical flash. His musical compositions are unquestionably some of the greatest in the "Standards" repertoire and like his playing it is easy to recognize them as Monk tunes for their originality and individualistic stamp. Oh, and like his music, he was original in the way he dressed :-)
http://m.youtube.com/watch?v=xAz_t2Ybvmc
