See unlike Diablo 3, I'd say the SC2's story is still salvageable if you pull Metzen away from it. There was almost no significant talk about the Hybrids, Samir (who, if you went to last year's BlizzConn, apparently made an appearance somewhere in SC2), or details about enemies of the dark origin. The real enemy remains nebulous & poorly understood. All we learned was that ONLY KERRIGAN can stop the "Evil", which doesn't really change the game, because, again, we know so little.
So given the true story arc, we find that not much really happened in SC2 to build towards that. It was a bunch of small story arcs that do significantly change the immediate lives of the characters, but nothing that even dents the main arc of this game.
In fact, SC and SC:BW were the same thing: the main story arc was introduced fairly late (Samir and Hybrids) and we learned little to nothing. Up till then, we thought the game was all about either killing/defeating/or brining back Kerrigan.
That isn't to discount Kerrigan's importance. She is THE keystone Character. However, if you played the Single Play beta at BlizzConn last year, you know that the demo starts you off with Kerrigan trying to rebuild the Zerg...which fundamentally creates a similar state to the end of the first game (albiet with potentially different intentions, but the same actions).
Now Diablo 3 on the other hand lol...that story was ruined
However, I don't think it was because of a bad story or bad multiplayer experience, or because greedy blizzard pissed off people who didn't want to use RMAH....Its simply a game that doesn't have that staying power for most.
If it was truly about greed, then Valve and TF2 hats is the clearest and most explicit money grab I can think of; Blizzard isn't even selling better "weapons" - they still make people find it, and its their choice to buy it.
When you sell 10+ million copies you expect many to stop playing once they beat the game. I had little to no intention to play much longer after beating Diablo 3, and that exactly what happened.
Is that a symptom of "A CRAPPY GAME" or "MP IS BORKED NEED FIXED?" or "RMAH SUCKS"....Nope.
For almost ANY game that has a 'beginning and end', do you expect people to keep doing the exact thing over and over for multiple of months on end? The only place we have that staying power is TF2, COD, CS, DOTA2, LOL, HON and select MMOs....games that rely zero on story and are focused on the collaboration with others (maybe not COD lol) to create that experience. Diablo 3 doesn't rely on playing with others to create new fun experiences. It relies on the concepts of slot machines to keep a certain select population addicted....and it works best on them
The core group of people will always stay and everyone else will leave, but it doesn't mean its a bad game. Besides, who, outside of the core group of people that love D2, still even play? Even then, I'd assume the vast majority that have wonderful memories of D2 don't even play D2...