The only thing I really care about keeping dry in the rain are my feet. Also I like having a cap with a brim I can flip down to keep the rain out of my eyes. Other than that, as others have said, it comes down to temperature management.

On a 200K earlier this month, it rained hard for about 25% of the ride. 65 degrees F. I was perfectly fine wearing my normal bib shorts and jersey, with a lightweight vest. Towards the end of the ride it got a little windy and cold, and I wished I had a little more, but getting wet from the rain didn't bother me one bit.