I was watching a demo of a first-person shooter set in the Middle East. The mission demo took about 30 minutes to play through, and involved the rescue of a wounded soldier who was trapped behind enemy lines.
I was bored ten minutes into the level. I'd played other shooters and didn't see the compelling reasons that would make this one stand out. Why not put in a plot twist?
For example, let's say you're trying to kill an enemy soldier, and then headquarters radios you that that soldier is a high-level defector possessing key intelligence information. He's only shooting at you out of self-defense.
You stop shooting, so he stops shooting. But now others from his side come after the both of you. So, now you've got to defend the guy you were trying to kill as you try to make it back to safety.