A staggering achievement; in Monsters & Magic, designer Sarah Newton has given us a rules-light game which runs off of D&D/AD&D stats, but uses its own 3d6 roll-high mechanic and narrative-style traits and consequences to use those stats in a completely new system. With that description, you might expect the game to be a horrible mess, or a fantasy heartbreaker ("let me just fix these minor issues in the most successful RPG of all time"); instead, it's an elegant, lean, fantastic reinvention which brings its own strengths to the party without throwing out any essential D&D-ness.
Here's a simple mechanics example which floored me on my first flip through the rules: instead of characters having movement scores of 12" or 9" as in AD&D (or 120 or 90 feet, as in D&D), they have Movement attributes of 12 or 9, rated on the same scale as Strength, Dexterity, etc. This means they have Movement bonuses or penalties that can be applied to action rolls where movement speed is important. Want to run a chase between a human with Movement 12 (+1 bonus) and a dwarf with Movement 9 (-1 penalty)? Simple! Pursuer and pursued make opposed action rolls using their Movement modifiers every round; the winner of the round's contest accumulates "effect points" equal to their margin of success, and this continues until one of them reaches a total of 10 effect points and escapes (pursued) or catches their quarry (pursuer). But this isn't a special set of chase rules, as seen in many games; it's just a straightforward and logical application of the game's basic action-effect and contest rules, which can be applied to handle almost any situation that arises.
Effects and consequences are the basis for the system; you use them to inflict damage in a fight, but that's the least of it. You can apply effects to disarm your foes, give them penalties (or yourself bonuses), move them, or pretty much anything else you want; it's a simple, open-ended concept with lots of good examples. My favourite thing so far is using an extreme effect (15 or more points of margin) to disable one of the special attacks or abilities of the monster you're fighting: you cut off the stinger with the poison, blind the eyes of the gaze-attacker, cut the vocal chords of the creature with the fear-inducing howl, etc. I can't convey just how brilliant an idea I think this is.
There are plenty of other changes. On top of D&D-esque physical combat, M&M adds a mental combat system (with its own initiative, hit points, effects, etc.) which covers everything from fear and shock to seduction or intimidation attempts. Characters have freeform traits (along the lines of Fate's aspects) and a very simple mechanism for their use; if you have a trait which is applicable to a contest, you add your level as a bonus to that roll (+1 for each additional applicable trait). Like many of the narrative-oriented elements of the system, the trait rules are optional, and easy enough to lift out of the game if you don't want to use them; that said, I think they're a great addition, and I'm eager to try them out.
This game came out in 2013, but I've only just discovered it. And I'm glad I did. If you like any kind of D&D-esque game and want to try a well-designed, flexible system that gives you that D&D experience deepened and enriched with more characterization and narrative interest, I strongly encourage you to check out Monsters & Magic.