Extremely configurable high-quality features
Reputation ranks
Ranks only ever go up; points can drop but your rank stays.
About reputation ranksAs a modpack dev, having mods be as configurable as possible is a massive help, if not a necessity for many mods. Quark really goes a step beyond and adds pretty much every single configuration possible, even adding a custom in-game menu for it.
The features Quark adds are comparable or just straight up better than other mods' implementations. The backpack is a favorite of mine. It's such a clean way to extend the player inventory, not just add a completely separate inventory (good for immersive packs, maybe not for tech or kitchen sink packs). Also love the totem of holding in place of other grave mods. It's simple, intuitive, and configurable. Quark really helps keep the mod list down and all it's features feel very balanced.
If you're on 1.12.2, highly recommend Quark RotN Edition for even more config features.