Some basic things,
Prop pitch is very important to regulate the temperature of your engine in most planes in il2 now (all that have a CSP). More so than throttle. throttle is manifold pressure, pitch is RPM, the higher the RPM the hotter your engine is going to be.
When cruising, people will often call out things like 70 pitch and 70 power for example, whilst leading a flight.
If you have an spare axis, prop pitch should be mapped to it. Along with a button to switch between manual and automatic, for the planes that require the override.
Radiator control is also very important, again for the management of engine temps, and needs to be something easy to use.
As is elevator trim and rudder trim, although these can be managed quite well on the hat switch on your joystick if using track ir.
Flaps need to be fairly easily accessible as well.
I'm sure more people will post more controls, but they are the basics to me, to be able to manage your plane.