Training Wheels ... from reading the docs, this is how I see it:
GAAP in IMC
Everyone is separated from everyone (IFR/Special VFR) There are no VFR aircraft in IMC
GAAP in VMC
No one gets separated. All get traffic info.
Class D in IMC
Everyone is separated from everyone (IFR/Special VFR) There are no VFR aircraft in IMC
Class D in VMC
IFRs get separated from each other.
IFRs get traffic on VFRs
VFRs get traffic on everyone
Besides the requirement for a clearance, the only real difference is that IFRs are protected from each other in VMC.