Rain is mostly a slight annoyance, however it can certainly be liquid death. For instance, the only negative about riding on a flat course in the rain is how unbelievably soaked you get. Not only from the rain falling but from the water coming up your wheels and onto your bike.

However, if you are riding in rain or right after rain down a steep hill you should be cautious as to not brake to suddenly. Also, make sure not to brake with just your front brakes or you could flip over your handlebars. Likewise with your rear brakes, if you solely brake with them to quick you will spin out and fall. I would suggest braking slowly with both brakes and make sure to slow down far up the hill so you do not slip and fall.