In the high speed system,
@1:
The comparator is generally multi-stage to increase the gain and enhance the resolution .
The amplifier is single or two stage to easy the compensation.
@2:
The comparator is not needed to be compensation, but the amplifier is needed.
@3:
The positive feedback is used in comparator to increase the speed, but the amplifier is used in the negative feedback to avoid the oscillation.
@4:
Basically, the amplifier is responsing with small signal, but the comparator is large signal response. Thus, the region of transistors is different.