If you listen to Fleas' progression from first album to latest, you'll hear the establishment of a truly innovative and thoughtful musician. As the previous poster so eloquently put ' he can really throw down, but he usually doesn't.' That to me is the mark of a true musician. Just give a listen to any of the tracks on Stadium Arcadium - he's subtle when needed, busy when appropriate, but always in service to the song. Another good insight into Flea is the interview that Bass Player did with him and Charlie Haden - the mutual respect between these two giants was inspiring.
FWIW - saw them live a couple years ago, on the S-A tour. What he pulls of musically while being the consummate showman was mind-blowing. And, he and John did some improv stuff that showed his off-the-cuff ability. Flea truly kicks ass