my $0.02:
-4MHz crystal doesn't actually mean anything because all dspics have PLL inside
-external eeprom is because dspic33's, pic24h's and pic32's don't have internal eeprom
-dspic33's have the ADC implemented in an interesting way: they can take up to 8 samples simultaneously.
-it has relatively non-shitty (as compared to other PIC microcontrollers) ADC @ INL+/-2, DNL < +/-1, gain error 1.25...3 and offset 1.25...2
-dsp functionalities such as 2 saturating accumulators and a barrel shifter (all 40-bits) and a hardware 17x17 multiplier and MAC were for sure very nice for building an accurate and fast PID regulator
How is the rectification of transformer output voltage done? I haven't noticed a bridge rectifier anywhere. Are they rectifying with single diode? Is the heater powered with AC or DC (AC would be my guess, because they seem to be using back to back N-MOSFETS for switching, and the two 3H7 optocouplers are most likely there to provide a proper gate drive for the MOSFETS).
EDIT:
-done a little searching for discretes near the optocouplers:
Y6W = BZX84C18 (18V zener)
Y4W = BZX84C15 (15V zener)
5D = MMBD914 70V 200mA fast switching diode
looks like a floating gate drive to me