I think the reason why people feel this way about Dragon Age II is that it didn't have a typical narrative structure ala Origins. A few things happen in the first act, which culminates in an event that doesn't really have anything to do with anything in the next act, and the process repeats itself two more times with the next act's events being foreshadowed in the previous one.
I think this actually is a sort of design choice by Bioware, and I find it interesting of them to use Dragon Age II to break the typical high-fantasy mold (because let's face it Origins was far from the fucking 'dark-fantasy' epic people were promised in the promotions). Regardless of whether or not the game is "good", the game is different, and the story is definitely more personal. It's all about Hawke's personal life and how Hawke deals with all the shit that's thrown at him/her. The setting of Kirkwall is merely a platform to display what Hawke does, rather than in Origins where the setting was the centrepiece of the story which the Warden so happened to make an impact on. That's what, I think, also influenced the way the narrative was framed through Varric's storytelling, because aside from the fact that Varric is the natural-born bullshitter of the party, he quickly establishes at the beginning (and in quite a few other parts of the game) that the story is all about Hawke's life (which Cassandra asked for,
) rather than Hawke's one overarching quest to go on a journey to location X to defeat singular enemy Y.
So while I don't necessarily consider it to be a 'spinoff', I can certainly see how people can see the game as merely a bridge to Dragon Age 3. It has only a few of the typical things that a regular run-of-the-mill high fantasy game would. Story structure is definitely not one of them. And I applaud them for being ambitious.