First of all, everyone needs to see the first Avengers movie, preferably after seeingCaptain America and Iron Man. Then, watch Avengers, and after that, Capt 2 and maybe a few of the others in between. After that, you care about all the characters in Avengers: Age of Ultron. That’s really all that matters with this movie.
The movie is an excuse to hang out with the characters. That’s all. The story is not really special, and the visuals are typical for this kind of film. (I will give it props for its excellent opening, which is very impressive, but I prefer the look of the current Daredevil series to the look of the Avengers films.) The movie is really a blast so long as you really enjoy seeing what the characters throw at one another (figuratively and literally) and you enjoy good bantering, drama, one liners, and character development. The story is, at the very least, set up so that the audience is very interested in seeing what the characters will do next, and how they’ll get out of tricky situations. It’s a fun summer movie, and that’s all it needed to be. I highly recommend it.