Minimum of rules for maximum of results
I want a system that doesn't provide me with minute rules for every possible action a character can make (never mind that it's an impossible task). I want a system that tells me how I can quickly come up with my own rules to adjudicate character actions. I don't want a system that needs tinkering, or a lot of rule memorization, in order to work. I want quick tools that are applicable in the maximum of situations. What RPGs so far might fall under this heading? The closest I've seen so far is Mouse Guard (based on Burning Wheel system), Wushu (terrific for one-session play, but a little too rules-lite), and (albeit with some tinkering) the New World of Darkness RPG from White Wolf.
Plug and Play
My ideal system does not require much (or ideally not any) tinkering to be used with a variety of settings and groups. I don't want ponderous rules for spelling out the nuances of all the individual species, classes, gear, etc. I want a system that focuses on character actions and decision regardless of what setting they might be in. The adventures of Odysseus are - despite the obvious differences - similar in structure and motivations to those of Jack Carter of Martian Chronicles, or many comic book heroes, or whatnot. That's what interests me more: the actions of the characters, their motivations, their adventures and personality. That's what I want the system to focus on, not tables upon tables of every conceivable minutae.
So easy your grandmother could use it
I'm not a dumb person, and I have a reasonably good memory, but I want the minimum of memorizaton required. If the rules could be fit on a DM screen in their entirety I would be ecstatic. If I could hand the rules to my players and they'd be familiar with them after 20 minutes of reading tops I would be ecstatic. I don't want a challenging tactical simulator (I'd play video games or tabletop wargames for that), or a super-realistic game. My criteria in this category is that the system should at no point slow down the game or take away the fun.
It keeps going and going and going...
There are many so-called narrativist and rules-lite systems that already do most of what I've set out above. Most of the ones I've looked at, however, suffer from a particular flaw - they do not support character progression with meaningful rules. The players are at heart stat-munchkins (even if they say otherwise). They want to see numbers change on their character sheet. They want to feel that their characters change in more ways than just their role in the story. And they want that progression to be meaningful and visible (again, even if they say otherwise). Most rule-lite RPGs I've read work great for short adventures or one-shots, but few offer this kind of progression that satisfies the players.
I've got the world on a string
The characters won't be doing much if there isn't a world to do it in. World building is a fun part of any GM's job and most of creativity and imagination goes into world building. All too often, however, I get frustrated trying to adapt a particular system to the world I have in mind. I look at the feats or skills or advantages or edges or aspects or whatnot that I would have to come up with for my world in order to make it work as intended. And therefore I return to my first point - instead of rules I want tools to help me make up monsters, people, effects, vehicles, gear, etc., on the fly without having to look through a book (and normally I'm lucky if it's just one book, usually it might involve multiple supplements) and thus grinding the game to a halt. That just ruins the fun and wastes time for everyone. Also, if I have to spend more time preparing for the game than actually running it, then I do not want to run it. That's why I don't run Dungeons and Dragons (of any edition) anymore.
Am I asking for too much? Eh, I'm sure I could get some good answers about people's favourite systems that purport to fulfill all my criteria, but to be honest - and arrogant - I doubt that they would. Eventually I'll build my own and inflict it on my players!