Definitely finish the floor with SOMETHING easy to clean and waterproof! We didn't. I can't figure out a good way to fix it now that wouldn't involve keeping the chickens out for at least most of a day, which would cause problems. So I'm stuck with a bare plywood floor, not the greatest.
Build on a good foundation and think about drainage if your climate is wet or the ground freezes. Think about which way the wind blows, and sun and shade as well as snow and rain. Seasons change!
Above all I would say think about predators, from snakes to mice and weasels, raccoons and rats. Seal corners, because losing eggs and hens to varmints is definitely nobody's idea of fun!
Install a light if you can and a way to keep your water from freezing if possible. These are all things I wish someone had told us! Good luck - and definitely have fun!