I think that when you find a gun, it should have a chance to have a very low durability (like under 100, or even under 50), which would force you to find a gun repair kit or weapon repair kit (or a new item) in order to repair your gun's durability. In fact, I think MOST guns should spawn with extremely low durability.
Another feature I think would work well with this idea is that your gun's durability should also affect it's ability to fire (ie, not jam when firing). A lower durability gun (past a certain threshold) would have a small chance of jamming, or perhaps it's accuracy would be slightly compromised, or perhaps the gun simply won't even fire (like if it's at 1% durability).
I think these kind of features would instill a lot more value in a gun, especially a well-maintained gun. It's a simple way to add "gun progression" without having to go the extra length of adding scopes, lights, dots, sights, etc. It would also increase immersion overall. For example, finding a weather-worn, barely working, beaten up .44 Magnum with 5% durability would be a pretty cool find, especially if you take the time to repair it to working condition — it would really feel like "your" Magnum. And if someone took it off you, you'd feel bad, cuz you spent a lot of time and effort to make it a gun worth having.
Now, I understand that this kind of hardcore gameplay element doesn't really fit in with the current 'arcade' direction that JS is going (with the ridiculously high increase in ammo spawns), but I still think it would be a cool idea.