For me (I know OD&D, 2E and 3.0E, mind you, not 3.5E and 4E):
FUN
1) Rules-light, or at least the option to go rules-light if it fits me and the players. Mechanics that could be easily stripped down to basics if desired. That also means minimal page-flipping during play.
2) Game mechanics that make sense and yield believable results (that is, believable within the setting's context).
3) On one hand a good amount of options to choose from both in character generation and during play, but, on the other hand, not too many options - and choices must be as clear as possible.
4) A balance between combat, exploration and role-playing (I like all three of these more or less equally).
5) A range of different complexity levels between different classes, or, at least, a range of different complexity levels between choices. Beginners, casual players, and people focusing more on RP than rules should be able to enjoy the game as much as experienced players and people who love complex tactics.
6) A simple core mechanic used everywhere - easier to learn and use the system that way.
7) A toolbox approach allowing maximal utilization of my limited prep time.
8) A system that doesn't get in the way of my handwaving and had-hoc rulings as a DM.
9) Fast character generation, especially when the players are experienced - the faster we get to the game itself, the better it is. On the other hand, players should have a range of choices to make to create their own characters.
10) Rules allowing play in one-on-one games or with a small (2-3 players plus DM) party.
UNFUN
1) Rules built with a particular play style in mind marginalizing all else. For example, rules you got to have miniatures to play well, or rules focusing on combat above all else, or a certain level of expected character wealth and amount of magic per level.
2) Over-complex rules, especially ones requiring multiple instances of book-referencing during combat (page flipping during play = double plus unfun!).
3) Significant amounts of prep-time or chargen bureaucracy wasting my limited prep time on number-crunching rather than on plots, schemes, interesting NPC personalities and cool bits.
4) Too much built-in setting assumptions in the core rules. I like to craft my own settings; a rule telling me that I can't have dwarf mages or gnome rangers is a rule I'm probably going to ignore. I also end up ignoring most core-book deities.
5) Character optimization. All character generation choices should be fairly playable; there shouldn't be character-option choices far superior to others or "legal" exploits. More so, the monster/encounter rules should fit most character choices, not just a party of a specific class mix with optimized characters.