The full (rain) monty
I do live in a wet, coastal climate, where it tends to rain during the winter (making it very green). I echo all of the above, especially the fenders and watching out for paint stripes, railroad tracks, etc. I also change the chain lube when it gets wet from White Lightening to something like Prolink, as rain can wash off lube. I also need to get a cover for my Brooks saddle. I find that my feet can get really wet, so those cool covers for shoes are a good investment, as well as a rain jacket and pants, and a helmet cover as well. I put on a rack for winter with a small trunk for extra stuff that might come in handy, like the rain pants. Because rain here is often accompanied by fog, I put some extra blinking lights. Once you make a little microclimate, as with running, it can be pretty nice to ride.
Of course, it is essential to stop for a nice steaming latte en route or afterwards!
The bicycle is the most civilized conveyance known to man. Other forms of transport grow daily more nightmarish. Only the bicycle remains pure in heart. ~Iris Murdoch, The Red and the Green