Why do surface mount mosfets have the heat sink pad on drain rather than source? (N channel)
For the same reason that bipolar transistors are heat-sinked through the collector. When the transistor is made, that's the big terminal on the back of the die. The source, or emitter in BJT's, is a mesh of little terminals interspersed with the gate or base terminals on the top of the die. It's simply the only practical terminal to connect to.