864 is a two-page, universal system designed to be lightweight but still crunchy.
The overall feel is very math-y / engineer-y, but it's not hard to grok, and it feels like it would be simple and fun to mod it into a more specific engine.
Normal rolls use d8s, d6s, and d4s (hence the system name,) and the standard TN on a roll is 8. Skills decrease the difficulty on your roll, and attributes can as well (although they are broad, expensive, and can increase the difficulty too.)
HP is 8, and attacks can wipe that out in a single hit, but consequences don't kick in until -4. Combat is simple, involves an action per character, and centers around passing rolls to hit.
Overall, I'm not certain whether 864 is easier to pick up than other short, crunchy unisystems. If you don't want to mod the game as you go, the core experience seems a bit swingy, and you'd be relying on the GM to make approaches other than just rolling for damage viable in combat. However, I think 864 might appeal specifically to designers as a foundation to be expanded on and customized. Tinker with it slightly, and I think you'll very easily get a game that fits your group, which is a cool niche. Pick it up if you get the chance to.
-xB(+/-) feels more complicated to decipher than "Bumps increase or decrease the difficulty tier of your roll"
-The permanent injury threshold is -4, but what happens if you go below that? Can you just go infinitely below and you take a permanent injury every time you're reduced further?
-Attributes costing twice as much as skills and being able to penalize rolls at the GM's discretion feels like it's better for players not to have them. Maybe when they incur a penalty, the player generates a point of a resource?
-Combat damage mentions PR, but I didn't see anywhere PR was explained.