I think everything I have to say has been covered. I offer my opinion on what is most important when it comes to riding in the rain:
GET A PINLOCK VISOR. All the anti-fog treatments I've tried work for a short time at best. I know of few things in riding more miserable than raising your shield a notch to clear the fog and then having rain streaming down both sides of the shield. Major distraction when you need it least. Waxing the outer surface of the shield is also good advice.
WEAR RAINPROOF GEAR. If you are NOT warm and dry, the discomfort will distract you. Uncomfortable is dangerous. Too much detail to get into here, so just one thought; have the coughs of your riding pants OUTSIDE your boots.
TRACTION IN THE WET IS HIGHLY VARIABLE. That diesel spill, tar snake, or kicked up sand which would have rung your alarm bell when the road was dry are now invisible. To presume you have 70% or more of dry road traction (even with good tires) discounts these facts of life. My voice of experience here is to take all backroad curves at what will seem stupidly low speeds. You can certainly up the speed when you have a straight piece of road. If there is any question of potholes (which will be very hard to see when they are filled with water) your best bet may be to ride down the middle of the lane.
BEING VISIBLE. I believe my white helmet, brighter jacket, and enhanced lighting on both ends DOES capture the attention of MOST cagers. But I don't trust these passive protective measures. In dry conditions, I try to remember I am invisible to a certain percentage of other motorists. In the rain, no matter your helmet color or lighting, I think it is best to assume you are invisible to all other motorists.
FORGET COMMUTING IN THE RAIN. OK, I wouldn't consider commuting on a bike on freeways near any major city in perfect weather. Nor would I live there. If you live in this environment, you still have to make a risk assessment.