I have done thousands of miles in the rain, maybe even tens of thousands of rain miles. I have always said, and tell my MSF students, "if you are prepared for rain riding it's not all that bad."
If you're not prepared, its a riding hell. Much of what is said above applies. Here are some other factors:
1. Right riding attitude about rain,..sssmmmoootttthhh in all your actions.
2. Scan/search even more and further down the road. See what you have to plan for well before you get there. This relates very high to traction issues and options. As said above, the painted surface got you more than did the rain. In some cases, dry painted surfaces can be just as slick.
3. Right riding gear makes ALL the difference. Dry, comfortable and reasonably warm lets you concentrate on the riding, makes scan/search much easier. BUT if all you focus is on how wet/cold/miserable you are, you're not focused on the riding.
4. Right gear also means right tires/good tread. Traction is the premium, don't do anything to squander it. If you're habit is to ride every last mile out of your tires, down to no tread, you WILL pay very hard consequences if you include rain riding. I found out the hard way and have a titanium rod in my left leg to prove it.
5. VISIBILITY is HUGE! Do anything you can to improve visibility in the rain, both for you and for others.
6. Distance/space cushion: back off create more space.
The actual load per square inch, and tire profile, on motorcycles is actually higher than that of cars on wet roads. So a motorcycle is less likely to hydroplane if that is a fear of rain riding. Doesn't mean you can just blast along, just saying hydroplaning is not as likely as people think.