You guys should have had the TF2 grief where a clan would lock people in saferooms and only let them out by asking them trivia questions. I get a huge kick out of those videos, they are really funny.
Well, personally, I think it all ultimately comes down to how Blizzard cranks it out. If the content in the SP (Since MP has been guaranteed to be fully fleshed out), is significant enough for a purchase by itself, then I think it will be the equivalent of a great movie trilogy, as long as it concludes more alike A New Hope than The Empire Strikes Back. By that I mean it has a conclusive finale, but clearly not the end of the story arc.
If the SP is lackluster and unengaging, then the community will lash out with the fervor of a thousand demons. Of course, that is true for any game, so I will keep being excited for a long-awaited sequel and of course judge the final product when it is indeed, final.