If you play through a game, watch the credits, say to yourself 'well that's that', possibly even uninstall, then read that the ending you played wasn't the best ending, what do you do?
Being there was two main quest lines, that should've hinted there was more than one ending.You could install a mod that adds new features or makes features available that was broken/taken out.
When Crysis comes out I'll probably get that.