Anyway, he posted the whole thing (only 6 pages or so) on youtube. A review would be a bit useless: the book is awesome and incredibly clever; go check it out.
But it got me thinking on how I would do a minimalist, equipment-based version of D&D.
3d6 in order. Modifiers are new-school style (+1 for 12/13, +2 for 14/15, etc.), but, to make things simpler, no negative modifiers.
Since abilities are so central, everything would be roll-under (roll 1d20 under you ability score to succeed), like Moldvay suggests. This would include saves, attacks and spells.
An ability of 3 would be very hindering regardless of the +0 modifier.
Natural 1 is a critical hit. Probably double damage. No need for "fumbles".
Here is how I'd use abilities to keep things nice and symmetric:
Encumbrance and armor
Encumbrance is central in such an equipment-focused game, and I obviously like the idea of encumbrance slots.
I'd be tempted to go one step further in the direction of videogames and give body, legs, chest, arms and head "slots" for armor, each with a +1 AC bonus. So a naked warrior with helmet and shield would get +2. A well-armored fighter would have greaves, bracers, body armor (chain?), breastplate and helmet, for a total of +5 or maybe +6. Shield would also count as secondary weapon.
However... just listing different types of armors with different weights, each giving a +1 point of AC per "slots" would be simpler.
Dexterity would give you an armor bonus... provided you have enough EMPTY slots as suggested in the link above.
Attacks are opposed rolls: if the attacker hits, the defender gets to roll "dodge", which will only be successful if lower than AC but higher than the attacker's roll.
Or just get rid of armor AC and do damage reduction instead. Seems fitting.
Weapons and abilities
Since the game is about abilities and inventory, I would make them interact a bit more: maybe limit weapon damage to you Strength score, so carrying a 1d8 blade would be less useful if you have Strength 5, add "finesse" weapons that use Dexterity instead of Strength, and so on.
Roll under Intelligence to cast a spell. Any spell.
One interesting thing about knave is that, to use spells, you need to carry spell-books. So "spell slots" are actually... encumbrance slots! Nice! However, you can only use each book once a day. Your grimoires get "bricked" after one use. Not ideal, IMO. How about magic points, to mirror hit points? Each use costs one magic point per spell level. Fail and lose twice that much. Less than 0 means terrifying death, or worse. Or maybe let the spell turn against the caster in that case - better have high Charisma then!
Magic points need SOME use for non-magicians. So maybe they are some kind of "luck points" or "willpower", and can be used for re-rolls, etc.
Charisma protects you from magic. Seems fitting with roguish heroes. "Physical" magic might allow you to roll DEX instead.
Tools instead of skills
Since there are no skills, 5e's concept of tools would fit perfectly here. A medicine kit instead of a healing skill, and so on.
Since we got rid of classes, why not go further and get rid of levels? Every significant "goal" achieved (a couple of short adventures, etc.) allows everyone in the group to augment one ability by one (maximum 20, although rolling a 20 is automatic failure anyway), and the person with the least ability score sum to augment two abilities.
For epic monsters, additional Strength (say, Strength 28 for an ancient dragon) could cause it to crit on an 8 or less.
No hit points?
Numbers would be lower across the board. A dragon causing, say, 1d6+9 damage would be scary for everyone. And with 24 (+6) Constitution, would ignore all attacks that cause six point of damage or less.
Spells would have to be nerfed as well; maybe 1d6+spell level damage would be enough, provided you can hit lots of targets with a fireball. A magic wand or staff could allow you to cast 0-level "missiles" doing 1d6 damage.
What's the point?
This is admittedly half-baked, but I was inspired to write down some random ideas for minimalist D&D and this is what I've got so far.
My ideal level of complexity is around 50 pages, like my Dark Fantasy Basic; however, the whole idea of Knave sounds so cool that I would be happy to try it for a few games.