Let me start by saying I know no comic book cannon, so I won't even try to go there.
The problem with media is the Buddhist understanding of impermanence. You are always changing and your experiences change you. We have already seen The Avengers, so now we are stuck with that bar of expectations. How to keep something old new, something already done undone and exciting. Sequels are extremely hard to pull off because you need to be new enough to warrant the experience, but familiar enough that people can just sit themselves back into it. No longer an origins film, we are watching an established set of heroes fighting together post The Winter Soldier due to the consequences unveiled in that film. How does it pan out? I think the feel of this is Avengers II to Avengers as is Captain America Winter Soldier to Captain America. It is a more "mature" movie. Some people may like it more than others.
Whedon manages extremely well. He knows the material, he is great at crafting stories, and unlike many action movie people, he is good at establishing characters. This film does this well, relatively early on. It is a crucial part to films like these. The key to super hero films is to realize that the viewer knows it'll be alright in the end, it is how to get to that point and for the viewers to care about the characters on the screen. Honestly, it is why Chris Evans is empathetical in Avengers rather than Fantastic Four. The party, which appeared unnecessary is being used to establish a couple major plot points to be used later on. I personally find that as effective film making, where the exposition isn't being told to you, but being shown. I'd also say, the pacing of the film was remarkable. I kind of kept waiting for the movie to end because some much stuff just continued to happen. I felt I got my money's worth about half way into the film.
One of my complaints about the film is the humor. It seemed to go all over the place from Whedon-esque brilliance to flat out campy comic book humor. There also seemed to be a little too much attempt at humor. Sometimes I felt like I was watching Star Trek seeing the jokes coming a mile away, and then sometimes the Whedon genius would pop up and hit you laughing. The early humor seemed too forced, but as the film moved on, it seemed more natural to the film. Ultron's humor bothered me a little, but I was able to write it off as he was created by Stark, so that could be expected.
The greatest thing about the film is it doesn't break the First Law of Science in Film. If you are going to butt fuck science, don't try to use science to justify it. He doesn't try to use science, and in general, it holds onto the largely forgiving realm of comic book plausibility. The one implausible thing I have is Black Widow. So there are a billion robots about to attack the Avengers. Everyone has a super power or a notable weapon, except her, seems like a liability.
The ending, really good. Looking forward to what is being foreshadowed ahead. Of course with Hulk and Black Widow... anyone that knows Whedon, those in relationships are typically doomed. There is little gain for Whedon for characters to be happy, when their misery can be exploited. Sad to see Whedon leaving, but I can't imagine how much effort he needed to put into the whole production. I hope he'll be around still to bounce stuff of him. I don't want the next Avengers to suffer the fate of X-Men III, from the looks of the potential replacement, we may be safe from that.
