Climate, for starters. But also light, ventilation, and flooring.
For whatever reason, people often install tiny windows and tiny doors, and it makes the barn like a vault. Then there is no air flow to reduce moisture and the lack of light is conducive to mold growth.
The best ones I have been in have large doors, large windows in every space, clear skylight panels in the roof, etc.
Dirt floors can hold moisture like a sponge. Concrete floors can sweat. Things like screenings with mats, pavers, or asphalt seem to work better for flooring.
Also, painting the concrete blocks inside and out seems to help a lot with moisture. The best barns I’ve known are painted with something that seems particularly thick and glossy— I never asked exactly what they were using, but I’m sure it can be researched. Untreated blocks always seem more dank.
Total change of topic, but injuries were mentioned several times. They can happen from kicking blocks. I cared for a mare whose racing career ended from a sesamoid fracture from kicking the block walls; I’ve known a few others injured as well. Yet injuries tend to be rare. I think just as many or more injure themselves kicking wooden walls. Horses seem to enjoy kicking wood more, too. I think the noise and vibration gives them some sort of satisfaction they don’t get from concrete. Or maybe it just hurts.