1. Take the D&D 1983 Red Box.
2. Clear out all the goofy, arbitrary things in the rules and replace them with reasonable alternatives (like explaining what happens when a spellcaster puts on armor).
3. Add a spell point system with several different types of magic.
4. Kill the thief's skill system and instead add general mechanics that let all characters do most of those things.
5. Add a vivid, imaginative setting.
The sample scenario in the book does a really good job of driving home the game's tone. If I had found this back in the day, I would've dropped D&D for it.
In a way, it resembles something that I really wish the retro gamers would at least attempt: take classic games and apply to them lessons learned through playing them. I had the vivid impression from DW that the authors either played a lot of early 80s D&D, or thought long and hard about its shortcomings, and set out to systematically address the game's issues while preserving the feel of a fast, easy, and digestible game.
I only have two complaints:
1. The assassin is b0rked, but easily fixed into a more thief-like profession.
2. I wish it had a sample dungeon. I really like the included adventure, but the text refers to the underworld far too often to dodge handing over a sample.
Tip o' the hat to all those involved. I'd really like to give this game a play.