Hello!
A few things that are very important are ventilation, and also to make sure they're not over crowded. For the ventilation, a cross ventilation, way up at the top of the coop, far above where your chickens roost is a good idea. (That way, they won't be roosting in a draft.)
Also, if you attach a run, make sure you use hardware cloth, because chicken wire can be ripped open by a predator. A roof over your run is great too, for shade, and it prevents attacks from above, like birds of prey.
In the run, I use sand, because then you can just rake out the poop, or scoop it, and the sand drains well too, if it gets wet.
I keep food inside the coop, and water outside. That's because if you have food in the run, and it gets wet, IT STINKS! BADLY! So you don't want that.
Windows are a nice accessory to a coop, to let some sunshine and fresh air in on nice days. You really need fresh air to be able to circulate, because stinky coops are not healthy for a chicken's lungs.
That's about all I can think of.