First and foremost I totally ignore the MP (maybe that's why I never got to become a true fan of Halo...), it's not my thing and even though I might play once in a while, it's never the reason I buy a game so I couldn't care less if the MP is great or not.
Single Player is what matters to me, I used to think it was a matter of how long it was and how good the story was. Lately I experienced a few games that proved me very wrong. That however will make my answer an extremely subjective and hollow one: the game needs to entertain me, I need to have fun with it. I recently bought Tomb Raider via Xbox Live because of the low price, I'm finding the game amazing in every way (and I was never a big fan of older Tomb Raider games), I also enjoyed the fuck out of GTA IV and Red Dead Redemption both lengthy games with a strong story (specially RDR). On the other hand GTA V is a game that I just can't seem to bring myself to finish, the graphics, imo, are amazing, the story is decent enough (actually, as far as Michael goes it's pretty funny), the world is incrdibly rich and alive, it has a level of detail I find amazing, the idea that developers created that city actually makes me wonder if we are not inside a divine entity's video game ourselves lol, but... I don't know, I just don't feel the game entertains me at all, I'm not having fun with it. Difficulty isn't an issue either, I love Dark Souls and that game is very hard, it's long, hard and the story sorta sucks (by that I mean it's not "story-centric"), but for some reason I have fun with it.
The games that changed my mind were Dark Souls, RDR, Hotline Miami and Mark of the Ninja. They have almost nothing in common, still they brought me back to when I was a little kid with a SNES and only a few games, I didn't care I'd played a given title, as long as I enjoyed it, I'd play it again, to hell with the story or length or even graphics, the only thing that mattered was the fun factor. With around 100 games or more in my X360 library, I sadly realize that i could've done without most of them, and wasted more time replaying my true gems (my, because it's MY fun factor)...