Wash stall should be cement or have rubber matting. The cement should have a drain or have a slight slant to it so that it runs off and doesnt pool on top....preferably to the back so that you arent having to walk through the puddle when you are done. Whether or not you have heated water is up to you. How cold does it get there anyways? Hot water is nice, but some people use more then they need too.....
I like a barn that has a lot of natural light if possible. My favourite barn ever was my grandparents 20-24 stall barn, two rows of stalls. They had stall partitions that they could roll into the aisle so that they could just drive a tractor right into the barn the whole length for a once/twice a year cleanout...two large doors at the each end of the row of stalls. The only downfall to those stalls was that they were solid partitions so the horses didnt have neighbourly interactions and less air flow.
I also like barns that have two large doors at either end of the aisles. It lets light and air in, and then you have two entrances into the barn.
The barn I ride at just uses those rubbermaid standup storage things....2 doors...5+ feet tall. If you have the ability to put in wood ones, they look way nicer then a clutter of mismatching ones. Lockers should be tall/deep enough to store one saddle, a bridle/halter, grooming brushes and maybe a shelf for rider stuff like their helmet/crop/gloves. The door should have a small mesh opening though so that nothing gets stinky! Lol. For a the school horse tack room, make sure that all the saddles are within reach of the shortest kid, or have a step ladder around. Extra girths are always a plus. We have a laundry basket for saddle pads and towels that need washed in the school tack room, as well as a small pail for water to wash bits when we are done.