You'll catch a lot of road grime in the face, so it's good to have some eye protection. Be prepared to clean grit out of your teeth, as well.
I train and race in rainy Portland, Oregon with Pro2Race or Krylon tires and KoolStop salmon pads. Some people prefer a file-tread on their tires for riding in the rain. Just remember to take corners easier than you normally would.
Wear a rain jacket to keep yourself dry and warm, and possibly keep your legs bare. Anything that holds the rain against your skin will just make you cold, so try to insulate your core as much as possible. Also, I like to wear wool socks so if my booties soak through, my feet still stay warm.
Good luck. Riding in the rain is fun, once you get passed that wet part.