Since I live in the rainy part of Oregon, I would never ride if I didn't ride in the rain!!
You do have to take special consideration of the footing when you ride in the rain [mud = slipping hazard] and you want to make sure your leather tack gets plenty of ventilation while drying [to prevent mold issues]...but otherwise it's really not bad!
I like to wear layers when riding in the rain - specifically, a wool sweater plus an appropriately light/heavy waterproof coat. The sweater keeps me warm, even if my coat gets soaked through, and makes for an altogether more pleasant ride.
Other thing: make sure all your electronics are going to stay dry! I ruined the screen on my old camera by taking it on a very rainy [like "buckets of water from heaven"-type] ride. THAT was a bummer. Haha