Running in a warm rain, however can be fantastic, especially shirtless.
One hazard or downside that can crop up however happened to me years ago. I was in the middle of a fantastic run a number of years ago while visiting my parents in Princeton. It was pouring and I was just carrying my drenched shirt at the point I reached campus. I looked down around that time and saw bubbles going down both legs. Apparently the last time I had washed them, my running shorts did not rinse properly so when the rain soaked into them, the remaining detergent decided to create a sea of froth down my thighs. I had to keep stopping to wipe them off with the drenched t-shirt.
Lightning is dangerous, wind makes runs awful unless you can do a point to point with it at your back the whole way.