+ Some extra stuff after you watch the credits roll. Especially if you actually have to wait and can't skip.
+ Some post death stuff - In Quake 2, the Tank Strogg would actually do shit to your corpse. In Halo, you hear some remarks about your death.
+ Dynamic hit response on enemies - Rage's enemies would react to where you shot them, made the shooting more fun. In Doom you can actually blow away specific chunks of demons depending on where you shot them. Very satisfying.