Since this is in the WW2 section I am going to assume that you only want our answers regarding WW2.
In an SP mission, I look for something with re playability and that requires you to cold start the aircraft. This can pretty much be anything from bombing to shooting down enemies, just please don't overdo it on the AAA :D
For MP, a good mission has something for everyone to do, it doesn't have extremely long flight times, and should have good base defence to prevent vulching. Something like a mass dogfight like in the old IL-2 days would be great, or have a bunch of ground targets that one side must protect, and the other side must destroy?
I'm just throwing ideas out here but I hope it helps you :)