I'd think you'd want a sand covering or something to absorb the poop though. When they scratch up the dirt in run it mixes and absorbs. Sand or some kind of litter over concrete would accomplish the same thing.
It shouldn't, I would personally put sand or some type of material to help absorb the poop. If you kept it bare then you woul have to wash it regularly. Another con to leaving it bare is it's less comfortable for the chikens.
X3 on the litter.
I have 2 buildings with concrete. One is the coop floor and the other is part of the run. I use shavings in the coop and straw in the run. Sand would work too. The main thing is that if you don't cover the floor, the feces sticks to the concrete like glue.