The drive train doesn't especially like the rain. It isn't the water, but the grime that winds up in the chain, being ground into the gears. Fenders help a little. But, what this ultimately means is you'll have to replace your chain a little sooner than you otherwise would have. Same goes for your cassette.

Wipe your bike down afterwards, and apply fresh chain lube after it dries.

Go a little bit slower, and plan on needing more stopping distance.