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!