Commercial pilot privileges more-so than knowing 121 and 135 (unless you know your DPE asks about that stuff). But if you know just the basics about what 121, 135, etc., is then you will be able to answer scenario questions by knowing what operations require an operating certificate.
Other stuff students seem to have problems with (In my experience):
Airworthiness (be able to prove your aircraft is airworthy), INOP equipment (whats required, what to do if something is inop, what you have to do to fly with inop equipment), MELs, Special flight permits.
Aerodynamics - theories of lift (bernouli and Newtons), adverse yaw, effects of CG placement.
Private pilot stuff people seem forget: Airspace, Required inspections, medical duration, currency requirements (BFR, landings to carry passengers, etc)... and anything else you may have forgotten since that check ride
.