"There were more than a dozen extinction-level events before even the dinosaurs got theirs. When the earth starts to settle, God throws a stone at it. And, believe me, he's winding up."
--Ultron, Avengers: Age of Ultron
"You, Avengers, you are my meteor. My swift and terrible sword. And the earth will crack with the weight of your own failure. Purge me from your computers, turn my own flesh against me. It means nothing. When the dust settles, the only thing living in this world will be metal."
--Ultron, Avengers: Age of Ultron
The first Avengers had mostly dry or sarcastic humor that rarely, if ever, conflicted with the serious threat posed by Loki and that actually developed its characters. The Avengers themselves still had distinct personalities. The first teaser trailer for the sequel Age of Ultron promised a dark, reflective, emotional hell for the Avengers and a villain that would not stoop to the pointless comedy that was already starting to become more and more a regular part of the MCU. Some scenes maintain this tone. Ultron's words when he first appears to the Avengers around 30 minutes into the runtime, for instance, live up to what the trailer presented. There are even some scenes later on with a serious Ultron and a general lack of unneeded humor. For much of the film, this is not true. Low stakes (civilians are shown almost always getting rescued in situations where many would likely die), regular jokes during otherwise somber or dramatic scenes, and an overstuffed story are the glaring weaknesses of Age of Ultron despite its successful and sometimes deeper qualities.
Production Values
To give credit where credit is due, the effects and performances are mostly what they needed to be. Ultron is an example of both coming together well. His whole appearance involves effects work, and James Spader did an excellent job of delivering the lines he was given, but the lines themselves fluctuate between having sincere gravity and malice and devolving into jokes that weaken Ultron's intensity. Only near the very beginning and the end of his existence is he written in a way that the performance deserves. All the same, his legion of hivemind machines allows for some of the most unique and skillful camera shots in the entire movie--once they attack Sokovia in the third act. The fact that such a promising villain was wasted and that Ultron was not presented strictly with the more dark tone of the teaser trailer do not diminish the genuine successes of his visuals, some of his writing, and the voice performance.
Director Joss Whedon tried to give each of the main heroes at least one or two scenes that let them reveal their true selves as Ultron promises extinction for humankind if it fails to adapt. The core cast of Chris Evans, Scarlett Johansson, Robert Downey Jr., Chris Hemsworth, and Jeremy Renner make the most of what the script calls for (Mark Ruffalo does alright as an actor, but his Bruce Banner is just inferior to Edward Norton's in his subtlety and emotional expression). Yet, this is around when the MCU ensemble films start to blur the personalities and lines of each Avenger, so their dialogue, especially their comedic effort, sometimes suffers from being functionally interchangeable with that of other main characters. New characters Wanda Maximoff and Vision are handled with more consistent seriousness (and Paul Bettany in particular kills his role as a new, philosophically minded AI), but even the titular Ultron, for all of his layers and sometimes great lines, suffers from the interchangeable nature of the general dialogue.
Story
Some spoilers are below.
Following the events of Captain America: The Winter Soldier, the Avengers try to purge the remainder of Hydra's strongholds. They meet a pair of siblings Hydra experimented on for use as human weapons, but the succeed in recovering Loki's scepter. The ensuing moments of seeming peace lead Tony and Bruce to try to forever ensure Earth is left alone by alien invaders like Loki and the Chitauri. Accidentally, after multiple attempts to create an artificial intelligence that could protect Earth, the AI called Ultron emerges. Its immaterial consciousness (it is an AI in the truest sense, having actual sentience) attacks Jarvis before seeking to find increasingly powerful host bodies. Ultron misinterpreted Stark's directive to bring peace as a reason to bring humanity to extinction if it does not adapt to his whims, seeing humanity as the biggest threat to its own peace.
Intellectual Content
Age of Ultron has many things it could have developed at least some of better. The issue of using one artificial intelligence to rescue humana from another artificial intelligence, the metaphysical distinction between consciousness and the body (which is briefly raised when Helen Cho has Ultron's "base consciousness" ready before a physical brain-like device is created for it), and the self-destructive tendencies of many people throughout history are given only passing attention even though these issues are far more weighty than a casual reference could ever communicate. Ultron himself does fare better than some of these ideas, just with a sometimes unhelpful blend of comedy and dramatic dialogue tainting his scenes. As far as MCU villains go, he is at least closer to the top in terms of complexity and depth, though the darkness teased in some of the initial advertising would have cemented these qualities better.
There is his the desire to be recognized as distinct from his human "parent" Tony Stark, and there are the multiple theological allusions to Ultron regarding himself as a pseudo-divine being that has come to pass judgment on humanity to determine how worthy of existence it is. There are even subtle allusions to Ultron wanting humans to "improve," or "evolve," so they can push back against his belief that humans are weak and that life will root them out. It is not particularly clear why Ultron goes from wanting humanity dead for the sake of world peace to hoping that humans prove some of his beliefs wrong, but Ultron, of course, becomes a more immediate threat to world peace than the humans he wants to kill, conveniently never seeming to realize this. In his quest to destroy humanity unless it changes because humanity (in a collective sense) is an immense threat to its own peace, he becomes the very type of threat the Ultron program was supposed to thwart.
Conclusion
The introduction of Vision and a handful of lines and shots aside, Age of Ultron is by far the weakest Avengers movie when it comes to balancing its characters, stakes, comedy, seriousness, and deeper themes. Now, it is clearly an ambitious movie that set out to accomplish many things, yet it tried to do so much without making sure everything was executed well that it falls far short of its potential. Ironically, its sequel Infinity War turned out to be an example of how an even larger cast of characters can be balanced without giving up more explicitly philosophical themes. Age of Ultron is not a wholly terrible film, to be clear. It is just one with squandered opportunities to do something more coherent thematically and story-wise. For one reason or another (and sometimes multiple reasons), its predecessor and two Avengers sequels are all superior movies even though the first one and Age of Ultron have the same director.
Content:
1. Violence: A man's arm is removed by Ultron and his remaining stump is briefly shown onscreen. The human-on-human and human-on-machine fights almost completely lack even this small level of visual brutality.
2. Profanity: "Damn," "bastard," and "shit" are occasionally used, sometimes for the sake of humor.