The main thing I dislike about rain riding is the reduced visibility in a hard rain. I still worry about someone not seeing me, or skidding into me. But - this is true even when I'm in my car, I find driving in a hard rain stressful and exhausting, particularly after dark.
If you live in an area where it rains infrequently, it will be slicker during the first few minutes of a rain, at least until the oil drippings wash away. A lot of "rain wrecks" occur in the first 5-10 minutes of a shower. I found this more an issue in Texas than where I now live, since Florida rains a lot and the streets stay generally cleaner. If a downpour just started, it's not a bad ideal to sit out for 10-15 minutes.
When I got back into riding a couple years ago, I took the MSF course, and it rained off and on during both range days. This was actually good, since everyone got to see how well you can brake and corner in the rain in a controlled environment. Maybe the MSF should always be conducted on a wet track?