Like most sequels, this one isn't as good as the original. There's plenteously of action and a lot of brash noises, but many of the characters seemed weaker and not as well-defined or as vividly portrayed as they were in the first film.
This film hits the ground running with the Avengers in full battle against a Hydra outpost where they see a couple of enhanced humans, Quicksilver (Aaron Taylor-Johnson of “Kick-Ass”) and his sister, the Scarlet Witch (Elizabeth Olsen of “Godzilla”) who are fighting for Hydra. Qucksilver is like the Flash and the Scarlet Witch has super powers, including telepathic influence over people's minds. Both of them have a grudge against Iron Man because of damage done by Stark Industries weapons.
Influenced by a mind trick initiated by the Scarlet Witch, Tony Stark (Iron Man, played by Robert Downey Jr.) attempts a dangerous artificial intelligence experiment with robots that goes awry, creating the super-villain Ultron, who, in turn, creates a robot army to destroy mankind. In another dangerous experiment later in the film Tony Stark and Bruce Banner (the Hulk, played by Mark Ruffalo) help to create yet another artificial life form called the Vision, who, with Thor's help, becomes an Avengers ally.
The Avengers, aided by the Vision (voiced by Paul Bettany of “Priest” who also does the voice of the computer system Jarvis) end up in a big battle to save the earth from Ultron and his robot army. This battle includes the emergency evacuation of an entire flying city.
Even though this film is almost as long as the first Avengers movie, it feels a lot more rushed and crowded with partly developed subplots, some of them ill explained. One such subplot involves an expedition by Thor (played by Chris Hemsworth) and Erik Selvig (Stellan Skarsgård) to some magic pool where Thor sees future events, including the creation of the Vision. This whole subplot makes little sense as presented. Maybe there will be an extended director's cut of this film someday that will flesh out some of these subplots.
One interlude that worked out well in the film is an Avenger's visit to Clint Barton's secluded home. Barton, called Hawkeye (Jeremy Renner of “The Bourne Legacy”) gets a lot more screen time in this film and makes the most of it. Barton's wife, Laura (Linda Cardellini of the “Mad Men” TV series) has some nice scenes with her husband and children in this film. It is a bit of normalcy in an otherwise fantastic landscape.
Another subplot involves a romance between Bruce Banner and the Black Widow (Scarlett Johansson). Even though a considerable amount of screen time is devoted to this romance, it doesn't seem to really go anywhere. This appears to be a plot element that will be picked up again in a future Marvel film.
The plot of the film is overstuffed. Either the film needed to be longer, or some plot elements needed to be dropped. There just wasn't enough time to develop all these characters and all these subplots. The actors who do get enough screen time, Renner, Ruffalo, Johansson and Cardellini, give good performances. The rest do the best they can with the time they have. This film gives ample evidence of how difficult it is to make a sprawling action movie like this and what a great job was done in the first Avengers film.