I think how light or how heavy an RPG rule set needs to be is really up to the needs of the players.
What does the group find fun?
Where does the game need to take them?
What kind of story do they want to put together?
For example, I don't think Russ (one of my friends and fellow players) would mind me saying that he likes AD&D. It just fits how he likes to play. Why? I don't know he would have to say. I have a feeling that having a ladder set up in the system that a character has to climb as they get more powerful (Ie levels) would have something to do with it.
Many lighter narrative games miss out on the whole “we are going to string these adventures together and your character will get better dynamic.”
Even Fate which is the game system of the moment in the wider RPG community, falls down in this regard with character improvement rules that feel a lot like an after thought.
It also seems to me that many games that look to be Gm'less (like our own game Phase Abandon) or light preparation games tend to lack direction. The story meanders off on it's own sometimes, and that can be great and it can be fun, but it might not be what very player wants every game.
Get me straight no one prepares less than I do. (another post for another time)
I think this is my actual point, every game system from D&D to paranoia to Fate to Dogs in the Vineyard, what ever are narrative games.
I think systems get chosen based on what interests the players and to act as a frame work to hang characters off of.
If players want one offs, Or a short fast and loose story experience, they pick one of the faster lighter more nimble systems. If they want a more traditional campaign experience they choose a more traditional game like Eclipse Phase, or any of the hundreds of other games that fit the bill.
There are even people out there who want to play crunchy games like Iorn Crown enterprises Role Master system, and insist they uses every last rule in it... (this I simply can't explain.)
At the end of the day the fun starts when the players start Narrating.
Narrating what the characters are doing.
Narrating what the characters are saying.
In that, all RPG games are the same, at some point characters have to do something, a player has to give the character life and in this light the rules set being used is a decision not much different than picking the pattern of your china. What the plates look like is not really that big a deal, they're all there to hold the good stuff, the food.
Yes system is important, it matters as it were.
If a designer writes a game that fails at meeting it's own design goals or strays away from it's design direction then yes it will be a broken game.
However all things being equal, players will pick a games that captures their imagination and match their idea of fun. After that the players have to put life into the game, and the game has to get out of the way. It's the one thing all RPG's have in common.