I have a lot of time invested in creating Wordpress templates. I want to release combinations of these templates along with different styles and Fancy Front pages as "Premium Wordpress Themes". What I need to know is what does "Premium" mean? What do people expect of a GPL theme vs. a Premium theme? Are there features that are considered required to be premium? Are there features that are in demand but considered "exceptional" i.e. not part of every premium theme? How can I tell the difference?

I have heard tounge-in-cheek answers that say that any theme that makes money is premium, but I mean to ask about what gives an outstanding theme it's quality. Why is it worth more? I am technically able to do many things, but as a lone developer with a family to feed, I can't afford to spend time on features that no one cares about. I have to try to isolate the things that people want. This is serious food and rent to me.

How can I get this kind of info so I can make my project successful?