The first important thing in the rain is to stay warm, not to stay dry. If you are riding vigorously, and true waterproof jacket will do little good since you will end up just as wet as if you weren't wearing it in the first place. At a more leisurely pace a waterproof jacket may be reasonable. Also, consider the weather that you will likely encounter. An afternoon thundershower in the middle of summer in the deep south is far different than a gusty, winter rainstorm further north. For the first, you probably don't need anything special, regardless of how you are riding. For the second, you will need much more than a simple rain jacket if you aren't working very hard.
For my own jacket, it first must be windproof. The wind will cool you off much faster than the rain, and remember that it includes the apparent wind due to your forward speed. Next, ventilation options like double zippers and pit zips are important so that I can keep the jacket on in many different conditions without getting overheated. I don't use hoods or pockets, so no need for those. Beyond that, I like simple, durable, non-bulky jackets. Some jackets have reinforced shoulders and waists for backpacks, and I highly recommend those if you will be riding with a backpack.
If I am touring in constant rain, I like to use rainpants and waterproof (non-insulated) shoe covers. This keeps my legs and feet comfortable and dry. I'll add leg warmers and thick wool socks as the temperature dictates. For short storms I don't do anything special for my legs and feet. In cooler or freezing weather I'll use booties and tights primarily to keep warm and don't need the rainpants and shoe covers.
Sean
