The main thing is to obviously make it big enough and secure enough for the chickens but also make sure you can very easily reach all the far corners to clean. With a scraper or brush if need be, but it's important to keep it in mind, in rain or shine. If it's windy, ours is a right pain to clean.
I made the external next box removable, as well as the entire side but didn't think what happens if it's lashing it down for days on end, so we just remove the nest box lid and clean it out that way. Also for collecting eggs of course. I built ours on 18" legs so we hang the feeder and the drinker underneath out of the rain. It's on a concrete plinth which helps keep it all clean.
I also made the floor removable through the side, but since we rarely remove the side, it doesn't get removed and I lean in to scrape it clean.
Fortunately I'm not so big and can lean through the hole to reach the corners.
It's thinking of the day to day things and making sure you can do it with minimum effort or it becomes a chore. Obviously every set up is different and you'll need to adapt to your situation but I can take a few photos to give an idea tomorrow (if I remember!). I'm proud of my roof. It's not quite St Pauls but it's all my own work :oops:
Also ventilation is an other thing sometimes overlooked. They need lots of it without causing draughts around them. Big holes right at the top that won't let the rain in with mesh over them.
Can't think of anything else at the mo.
Rob