1. Comfortable on my head
2. The more airflow the better
3. If I'm planning on riding when there's rain, I prefer Specialized Helmets, because they have a big port in the front of the helmet. I can wear a rain cover on the helmet but tuck the front into the port and keep the back up a bit, which leaves a lot more air flowing over my head while riding. My head gets a little wet that way, but when it's kinda warm it gets way less wet than the other two options - no helmet cover is obviously very wet, but a helmet cover with no airflow also gets my head soaked from sweat.

#3 is not important if you're not planning on riding in the rain though.