TrotimMore people willing to seriously playtest them, repeatedly, in early stages who then don't leave only toxic feedback.
This unfortunately is one of the main reasons why custom maps are quite sparse in the community, NA are more open to new maps but EU are incredibly tight lipped about playing and trying out new maps which is a really big shame.
I remember in CSS and 1.6 there were loads of new maps play tested and tried, when you searched for a scrim the other team wouldn't bitch and whine about having to play a certain map.
In regards to designing maps themselves, quite a few custom maps are incredibly good but I feel most are all lacking in good last points in particular, most are very much the same in some shape or form which is disappointing. I still think for this reason badlands is a really good map as it's one of the few maps that has a closed last point that really creates some intense last defense pushes, most these days have rather large open last caps that make for a very flat and bland last push.