For me, a system that keeps everyone on the same playing field - Equal balance for all.
I don't like systems where you can pay for content that unbalances the game. This is a huge one for me.
I understand hardware costs money, I understand that people need to earn a living, and programming is a job and this can't survive forever as a "hobby".
If income is required, a trivial amount (Even $2-3/month) just to cover the bills and costs. Maybe free users get no 50+ abilities like incarnate? No additional pay options.
I would like development of new content, new archetypes and powers, but not if it means going more than $7/month. I have 3 people in my household who play this game.