I may be in the minority from other critics, but I actually enjoyed the first Avengers movie (which is actually one of the very first movies I reviewed on this blog, and you can find the amusing original here). It was a little messy, but I had fun watching it and director Joss Whedon did a good job of juggling the vastly different characters and threading together an overall story. The challenges of bringing together such a diverse cast were addressed and choreographed fairly well. Unfortunately, the old problems are far too apparent in the Avengers sequel. It's perhaps understandable and excusable for a group of heroes to squabble and act like children in their first outing together. But although they start the second movie in harmony, it's far too easy to pull them apart and have them at each other's necks like a Little Rascals routine, only with the whole world at stake. The Avengers are clearly not greater than the sum of all their parts.
Age of Ultron centers around a conflict that arises from its own ranks when Tony Stark (Robert Downey Jr.) attempts to channel the power of the Chitauri Scepter into the ultimate peacekeeping A.I. For Stark, he's tired of repeatedly saving the world and perhaps a little jaded from his last jaunts of sacrificing himself for the good of all. The attempt, of course, backfires to create an ultimate nemesis in mechanic A.I. Ultron who decides that the way to protect humanity is to destroy it completely.
Most of the conflicts that arise throughout the movie are basically a problem of communication and all the Avengers trying time and time again to act independently. The resolution of the movie is unsatisfactory in addressing this issue and it's tiring for the audience to constantly be deterred by their pigheadedness. It's worse than bringing together NBA players for Team USA at the 2004 Olympics. I constantly wanted to reach into the movie to knock their heads together or enforce a time-out. It makes it seem like all the character building in the first Avengers (and subsequent movies) has been worthless. The story lacks as much cohesion as the team, as Whedon apparently had consistent clashes with the studio and producers, even having to sacrifice one story arc in order to protect another that fleshes out Hawkeye's (Jeremy Renner) character. The process of battling with execs as well as the exhaustion of maneuvering so many disparate characters actually contributed greatly to Whedon's decision to give up the Avengers directing mantle after this movie.
Whedon's not the only one bringing things to a close, it seems. Much of the movie, interestingly, seems to explore what it means to retire from a life of saving others and if there is a right time, or if those that have the power ever have qualifications that allow them to pass on their role.
Avengers is an entertaining jaunt and choreographed well enough that the explosions and fight scenes aren't exhausting on the audience psyche. These combination stories are usually more about seeing a huge cast come together more than anything else anyway. If you take it as a fun excursion while allowing that character development and story are thin due to some oversaturation, it can be an enjoyable outing.