I sat down to play Mass Effect 3's new ending content yesterday, only to realize that I had never completed the game with the "effective military strength" score required for the "best" ending. So, I played multiplayer, and it was awesome.
Bioware wanted all your decisions to matter, for everything you do in the Mass Effect world to impact your particular ending. Their execution on this has been a matter of hot debate.
Everything you do within ME3’s single player contributes to your military strength score. Mine was around 7000, which is about as high as it can get in a completionist playthrough. Everything (multiplayer, mobile stuff) you do outside of ME3's single player contributes to your "Galactic Readiness Rating," a percentage (defaulted at 50%) by which your military strength score is multiplied. These two combined make your "effective military strength." In order to get the "best" ending (arguable), one must exceed an EMS of 4000. I had 3500.
In short, you must play multiplayer to access a (pivotal) 5 second scene following the end cinematic.