The only real disappointment I have with MM4 is in the UI. I prefer unskinned operation, despite the ridiculous lack of features of the unskinned player (no Album Title or Year display, no Rating field, no "Stop After Current" button, and generally ugly).
But MM4 seems to me to be slower to react to mouse clicks and to redraw panes than MM3 was. Obviously, that's subjective; I don't have the resources or inclination to measure this. I don't know what language MM is coded in; I fear it's Visual Basic. I think the UI could be a lot snappier if it was rebuilt.