Some painted lines on the road can be slippery, as well as anything metal like sewer caps, so try to avoid them. Wear a cycling cap under, or visor on your helmet to keep your glasses dry (if you wear them) and bring something to wipe them with. The worst part of riding in the rain for me is when I can't see, and the bike cleaning, of course. Otherwise, riding in the rain is really fun. Oh, and bring extra socks in a zip-lock bag! If it's raining hard, your shoes can fill up with water, and that's pretty unpleasant. Afterward, stuff your shoes with newspaper to help them dry faster and keep their shape.
'02 Eddy Merckx Fuga, Selle An Atomica
'85 Eddy Merckx Professional, Selle An Atomica
'10 Soma Double Cross DC, Selle An Atomica
Slacker on wheels.