Road lines and metal parts will be slippery. Very slippery. Avoid braking or leaning on those.

Pools of water might hide a pothole, so if you don't know the road well, avoid pools of water.

Brakes will work slightly less well. Kool Stop or Swisstop Salmon pads could help with that.

Apart from that common sense, no reason not to ride in the rain. I ride all year long. Make sure you have visible lights, make sure the bike is lubed regularly.