They are SLICK! the most innocent acceleration will send your rear tire spinning. A mild hint of throttle in a corned can throw your back end out and dump you on the pavement (But incidentally, wet pavement wont hurt as bad falling on it. It will still hurt). Brakes lock up easy as well. Front wheel will try to wash out to one side under moderate braking. If it does, let go of the brake and reapply. Try using the rear brake more. If it locks the conciquences arent as dramatic.
In addition wet disks present an interesting danger. As you come down on the brakes, the water lubricates the pads, thus braking is almost non existant. The common reaction is to apply more brake, which will quickly dry up the rotor and then lock the wheel up. If you do ride in the rain, try applying the brakes just a tad every minute or so. You're not looking to slow down, just dry up the rotor.
Other dangers include low visibility, fogging visor, and increased chance of tire puncture since sharp rocks and nails are now lubricated and can cut thru rubber easier.
In short, avoid if at all possible.