Having a game that's easy to modify also spurs player interest. Be it the ability to replace models, add sounds or just simply change a texture or font. It allows the community to make what they want, and a large majority of game developers started as mere modders.
Being able to charge up your attacks adds a slight bit of depth - and gives you multiple ways to beat an enemy
Killcams haven't really been used in many games outside of Call Of Duty, which is a shame. They show insight towards your death, your enemy's whereabouts at the time, and gives you adequate time to realise any mistakes you may have made.
Portals! Honestly, it's understandable why they aren't used outside of Arena shooters as well as the Portal series, but hell, they're awesome.
Having a developer console in the game is simply amazing. It can free a player's experience by letting them change settings they couldn't before, and change a few values to see how a game can run with things like low gravity or double damage with certain weapon.
Powerups, we all love them - Whether they give extra damage, high jumps, or invulnerability. They've been proven to work in both Single-player and Multiplayer games.
Secondary attack modes. Whether it's the airblast from the TF2's Pyro, or /every gun/ from Unreal Tournament. They simply add more depth, which is beautiful when not complex.
Bunnyhopping - Though technically a glitch, some games have used it to their advantage - And it's a basic technique which can enhance gameplay a lot. The problem is, is simplifying it a la Quake Live better or worse for players?