Four origin stories in one movie?
I didn't see it that way.
The twins: the origin of their powers was not shown. We're just told "Hydra experimented on them". Their story about the Stark bomb is necessary to tell us where the characters are at mentally, just like we know the motivations of the rest of the Avengers.
The origin of Ultron is the whole point of the movie, and the origin of Vision as created by Ultron has always been part of that story. It's not just an excuse to fit Vision in there, it
is the plot.
So in retrospect, I can see why you'd say that - but for me, watching the movie, it never felt that way.
In the end, Ultron felt more like a misguided child than a supervillain.
He's often been written that way in the comics too. It's part of his character in many stories. They may have taken it too far, and the petulant brat act is a Spader specialty, but I didn't find it out of line. (Not a knock against Spader, he was amazing, but his acting style does tend to emphasize the "spoiled brat" aspect of many of his characters over his entire career. The script played to that strength of his, and Spader ran with it like you'd expect.)
While it may not have been as good for those who only see the movies, I thought AoU came closest so far to the tone of the comics. Tony Stark was the closest yet to his comics characterization (especially the small things like the little "sorry" when he knocks Hulk's tooth out). Vision was also very much like the comics characterization.