This question was asked on Reddit:
There are plenty of rules-lite games (and games that are light on rules) floating around out there. What really makes one rules-lite, and what makes for a good rules-lite game? I want something easy and simple to run one-shots with every couple of weeks for some new gamers.
It's my personal opinion that in the last thirty-six years[†]("Yeah, it's been that long.") a majority of the role-playing game community and industry has lost sight of what the category role-playing game[†]("In the context of pen and paper games.") means. It has been wrapped up in this oddly self-referential definition that could describe just about anything fun at the table. In order to answer the question "What ... makes ... [a game] rules-lite, and what makes ... a good rules-lite game?" I believe pinning down the definition of role-playing game is required.
According to the US Copyright Law[†]("which is by no means a complete definition, but a start") a game is loosely two parts: Mechanics and Content. The law states that a person can copyright the Content[†]("materials, diagrams, art, and correction of words (but not titles or specific names)") of a game, but not the Mechanics[†]("resolution of rules, procedures, or protocol"). This gives us something to work with: A game is the sum of it's Content and Mechanics.
What then of role-playing? Luckily this has a very simple definition:
role-playing: Noun [role-playing (plural role-playings)]
Action in which a person takes on a role (as that of an actor) and pretends or acts out being that character.
So in conclusion we have three parts to a role-playing game: Content, Mechanics, and Role Acting. The combination of these items comes together to complete the experience we like to call Role-playing Games. This doesn't quite answer the so much as clarify the question, reformed as such:
How do we define a role-playing game with light mechanics? What is the definition of a good role-playing game with light mechanics?
Now we have to dig deeper into what constitutes Mechanics. As defined above the US Copyright Law lays out that Mechanics are listed as, but not limited in scope to, the method or methods for playing [the game]. Much like a tool the mechanics of a Role-playing Game is used to make gaming easier. For instance instead of arguing over if the Cowboy shot the Robber we can find out through the protocol defined in the Role-playing Game.
In short: Role-playing Game mechanics are intended to make role-playing and gaming easier. Extrapolating from this we can then determine that a mechanics are the stepping stones we use to get from point A (a conflict) to point B (a resolution).
Ever since we've started playing role-playing games we've attributed arbitrary weights to how many rules, or stepping stones, we're required to bypass before we get to point B. Typically the longer it takes for the players to resolve the conflict the more heavy a role-playing game's rules are. Conversely a game with less conflict resolution mechanics is considered to be lighter. Alternatively we also consider lighter to mean easier to understand.
Since each group works differently it's ultimately impossible (or at least improbable) to define a set of mechanics as heavy or light universally. Recently (as in the last ten years) I've seen more and more games defined as light simply because they are smaller in material size than the mainstream games (Dungeons & Dragons). That said people who play role-playing games still want a game and in that sense the answer to your question (reformed) is:
A role-playing game with light mechanics is a role-playing game that requires less effort to resolve conflicts. A good role-playing game with light mechanics is a role-playing game that contains mechanics that avoid getting in the way of playing the game without removing the game aspect.
I hope that helps answer the question.