Diode and magnetron are also frequent failure causes. The magnetron is easy to test with a resistance meter. Should be low ohms through and infinity to its casing. (all cables removed of course)
I believe a new magnetron was fairly expensive, and I'm not by any means good with electricity (beyond some very simple car stuff), so I didn't even bother trying to check it. We kind of hated that microwave anyway, its beeps were so annoying.
It just stopped heating things up. A bit of research suggested that the most common failure was the capacitor, which was like $10, so I figured why not? I was going to have to take the broken microwave off the wall whether I could fix it or not.
Microwave repair can actually be quite dangerous if I understand correctly, as they can hold a residual charge which can be very harmful if you short it.