Really, it's whatever you like. Some people will pick based on quality, some will pick based on artistic style, some will pick on what "speaks" to them, some will pick on any other number of things.
Once we get to voting, then all the pictures meet the requirements for the contest so it's whatever you want to vote for.
I find myself voting for pics that are more creative and that quality doesn't factor in as much. I feel like voting for quality implies that if you don't have a nice camera or aren't a more serious photographer than you don't have a chance in any of these contests. I want as many people to participate as possible regardless of the quality of their photos or cameras or whether they have any technical expertise or not.