I've played the last scene of the MW3 campaign multiple times trying to get the game to acknowledge that I have finished. I've let the credits run to the end and then selected "Continue" (the only choice) which takes me out to the main menu where it shows my campaign is only 50% complete. I have "resumed" multiple times, trying at other times to "Save and Quit" following the last scene, but ultimately this has become my last checkpoint to which "resume" returns me each time. How do I resolve this? XBox360 is my console. All updates installed. "Save to" location is the hard drive with 200+ Gigs of free space.
You are done. Start a New Game at a higher difficulty level and get all the achievements. That's the reason why you're 50% complete.
Check your achievements on your Xbox Guide by pressing the Xbox Home button on your controller, go to Achievements and look for the Who Dares Wins Achievement. I'm sure you have that. If you don't, then you definitely have a problem.