I prefer worlds of fantasy. Energetic players who are creative problem solvers are the best players. When I DM, I try to maintain a good balance of role-playing to combat, which is afterall what really makes the game exciting. I don't care for people who are wrapped up in the rules and rules exploitation. Nor do I care for rude, loud or uncontrolled outbursts of profanity.
Most of my experience is as a DM/GM. I have played many systems but prefer fantasy. I'm in the military so I haven't gotten very many chances to run or play in a good solid campaign recently. Usually by the time the plot has thickened and the pieces start falling into place its time for somebody to move on. Usually me. As a family man I try to keep my gaming to every other week.