I don't miss the Ki power source. It was always a one-class pony.
But I wish they had made monks Martial instead of Psionic. Psionics developed a very distinct flavor over the course of 3e (it featured crystals and bending spoons and some pretty freaky stuff involving ectoplasm) and I don't think monks fit with that very well.
A Martial power source fits better with the source material for monks (they are even called MARTIAL artists). In the kung-fu and wuxia genres, martial artists use their extreme training to perform feats bordering on the supernatural. I know some people would not like the idea of a Martial character flinging fireballs, but I'm fine with it, especially at mid to high levels, and in a fantasy setting. At least, it seems to fit better than Psionic to me.
-- 77IM