Part I, buying is going to be a mix of the five characteristics.This is really a two part question: "What gets you to buy and play an RPG?" And: "What gets you to keep playing the game?"
Here is my take personal opinion on it and I liked to hear others' point of view.
Concept. Lots of games have awesome concepts, and I think this is the big attention-getter. "This is like x but with twist y." A cool concept can get someone to download a pdf or even shell out a few bucks, but quickly looses its appeal if the game doesn't follow through.
Mechanics. The crunch. As game designers, this is where a lot of time is spent. IMO, it's generally not a major selling point, but it can make or break a game. I've seen cool concepts crash and burn with flawed/clunky mechanics (or sometimes just poorly written rules). Generally, well-made mechanics are what makes me keep playing a game after the first try.
Fluff. The concept is big, but the world meat is in the fluff. A fleshed out world that a good GM can share with their players is a lot of fun. This is hit or miss with me, but when it hits it hits big.
Fluff-chanics. This is what I call it when a game stats out the rules for doing something that other games usually just let the game master wing. There is a chart for what happens when you go to hire a mercenary, a social resolution matrix, and detailed romance system. To me these are just passing novelties that don't really add much.
Art. Sad but true. (Maybe?) People pick something up if its got rad art. I've done it.
The most important to me are concept and mechanics. I want the game to do something specific (even if I play GURPS, I'm playing one of many genres from the buffet) and do it right. The game should be something I could grasp and have rules complex enough to try several character builds and not have to do crazy things to make a competent character. Concept is also important, because I'm shorter on time than I used to be. Who am I playing and why are important--I want to be immersed in the world. That makes fluff #3. I can ignore some and make my own, but I need a world to play in. So, make fluff that gives me enough to make me want to play. Fluff-chanics and art help out in building the world. Art's probably the least important, as I'm not a real visual person, but the art should fit the mood the game is trying to convey. If it's cheap line art, that works fine if it shows things well enough so I know the world.
As for keep playing, the game needs to keep the other players happy (I usually GM) and let me use my imagination. Robust mechanics and enough content to play with are key. I also want a sample adventure so I can figure out how the game is supposed to be played.
While a game designer cannot tell me exactly how to play their game and account for all situations, I'm looking for a good melding of mechanics and content. If I'm supposed to be in a post-apocalyptic world where resources are scarce, I want the game to reflect that and make me desire to play a character in that world.
I'm pretty set on games for right now, but could definitely use games which promote less violence and have better social resolution matrices than the old D&D encounter table. . .