Assuming safety is a given, then cost for sure.

Following recommended safety guidelines when buying a crib.

Longevity. I like the convertible ones that you can use until they are 4 or so. You get better vallue for money that way. So then I can spend a bit more to get something nicer rather than a cheaper one I will replace a couple of times.

Order of importance!: #1: Safety #2: Cost/functionality #3. Trend

does noone care about the baby's comfort?

No way! I just put my kid on bricks! All in the name of style! ;)

I think the reason its not mentioned is that for many cribs the mattress is separate so it would be a separate purchase. We got a high end fancy schmancy mattress that seemed comfy.

I'm with @shashashelby on longevity (not the bricks, hehe). A crib that is convertible is a huuuge plus in my book.