Mical:
An important function of the LED driver is to keep the junction temperature of LED emitters low.
This can be done by reducing heat dissipation, and to provide passive or active cooling inside the LED bulb.
Also, the Driver should last as long as the LED emitters. Thus, it shouldn't contain electrolytic capacitors.
Most importantly, the Driver must be so cheap that it doesn't add significant cost to already expensive LED emitters.