I'd like to run or join a Fantasy campaign. Detailed characters, histories, motivations. Player initiated stories interwoven with GM ones. I like systems that feel "open ended," where your character becomes unique from any other. I like adventures that are open ended, where a GM presents an interesting dilemma and then lets the players do what they do best. I'd like player choices to be more along the lines of who's side should I join in battle rather than left corrodor or straight ahead. My favorite system is Ars Magica which really supports the above. I think combat is necessary (for me), but the true joy of a good combat scene is a wonderful story leading up to it. I'm looking for a long term campaign filled with players who are excited about their characters. I like a mid-level of party unity - its a little boring if all 5 players act as a single hive-mind. Nor do I want them actively trying to assassinate each other. But general unity, peppered with needed compromises, and occasional character driven differences is the spice of RPGing.
I've played D&D from 1st ed to 3.5. I then found Ars Magica which I've played for about 10 years. Of course, along the way, I've tried many systems for individual adventures. I'd love to get into a 2 session/month long-long term campaign.