The main problem I found with the movie, is the same reason so many other sequels sometimes aren't so good -- it HAS to be bigger and better and louder and more exciting and have more explosions and so forth and so on.
I saw three main action scenes and I think they went on a bit too long. Now, I realize with SO many characters, each one has to have his or her place in the sun to showcase their powers, but that was the problem - too many heroes.
I think they should have stayed with the core group from the first movie and left out Pietro and Wanda Maximov. I liked them, but they took time away from the others. Perhaps they could be included in some other Marvel movie.