For about two years we had a stationary coop with an 8'x4' footprint. I didn't like the condition of the run, though; they scratched the grass away in no time, and the ground was constantly covered in poop.
I started designing a coop with an 8x4 footprint that we could move around, thus providing fresh grass for the chickens to scratch, and keeping the ground from being covered in poop right away.
This is what we ended up with. I'm not a carpenter, so I definitely did some things wrong (I could point out the flaws, but couldn't tell you for sure how I'd fix them), but we've been very happy with the coop. I couldn't figure out how to drag it across the yard with my truck without digging into the grass, then I found some $3 water skis at ReStore, which solved that problem (although it does look a little ridiculous.)
The doors slide in and out because I didn't want to spend a lot of money on hinges. We're very happy with the way they turned out.
This design offers plenty of ventilation; the droppings dry nicely and are easily removed. No condensation, etc.
I did a lot of research on size (chickens per square foot, etc.), and figured that we could have a flock of about 20 birds and they'd be comfortable in this coop. I wouldn't feel good about keeping them in here all day every day, but on days when we're not able to let them out to graze, they seem to be reasonably happy with the amount of ground they have available.
Just thought I'd share with the group...