Well, water and a whole additional set of hazards to watch for. I know that many riders will not ride in the rain. I know that my wife questions my sanity when I start putting on the gear to ride to work when it is pouring down rain outside, but the thing is, I choose to ride as my transportation. It is unreasonable to say I am not going somewhere just because it is raining. So I ride in the rain.
Yesterday was an excellent example. I knew the forecast was for thunderstorms all day. All this meant was that I dressed accordingly, slipped on the rain gear and out I went.
Sure, once on the bike, things are a little different. Speeds are a little lower. Turns are taken with more emphasis in maximizing traction. Really, just a normal ride with some extra awareness, except for one thing. Other drivers behaviors become even more erratic. This is expected, and is just something to be dealt with. I find it very interesting that other riders in the area intimidated by riding in the rain.