Seems like every review opens with "I enjoyed it but...." and who am I to toss aside a week-old tradition? Because Age of Ultron is entertaining in a loud-fast summer blockbuster way but lacks much of Whedon - where's the first film's humor and sense of real conflict? Where's the feel of character and that smart dialogue? I also find it odd to complain that it's too much like the comics but really that's more an issue of book elements that work there but weren't translated properly to film. Seriously, levitating an entire (if small) city works on paper but seems a bit silly here.
When we left the Avengers at the end of the first film they had separated and Fury stated they would be back when we needed them. That sense of desperation (and religious overtone) is absent - the new film opens with them having been together for a while though it's unclear why exactly. Yes, Hydra clean-up duty but is that all? By the end, we're shown a way for the franchise to move forward even if the current actors leave. Comics readers are familiar with frequent line-up changes (or for that matter multiple concurrent Avengers teams) but it's not certain if movie audiences will be as open-minded.
Ultron appeared to be a good choice for an opponent - smart, tricky and almost impossible to defeat. On film though he becomes more a generic villain. Why does he do bad things? Well because he's the bad guy of course. There is reasoning - he's programmed to defend the planet and thinks the best way to do that is to eliminate people - but it's tossed in. Consider how much more effective it would have been to have a scene where the newly created Ultron fights alongside the Avengers and we see him reasoning along that path. Instead it's just a given and unconvincing, perhaps one reason he embarks on such a bizarre mad-scientist scheme. (Not to mention that Ultron's plan is remarkably inefficient and would take thousands of years to kill humans but at least it's visual.)
Ultron also becomes something of a faceless entity and the struggle against him is either technobabble or smashing robots. Think of the first film where numerous people argue with Loki, try to reason with him, bring him to a different view, trick him. The closest this film comes is the Avengers trying to guess what Ultron is doing - hardly the same and resolutely undramatic. (Just as the sudden appearance of a helicarrier at a needed moment is sloppy writing.)
The film also barely exploits Ultron's biggest strength in the books - that he's an AI and since independent of a physical host nearly impossible to eradicate. The film briefly has a bit where Ultron jumps bodies but otherwise he's in the same one throughout. It's easy to see why - because fully using his AI state not only would turn the film into a computer hacking story where most of the Avengers have nothing to do (even if set in a Tron-like visualization of cyberspace) but that would make a sense of closure at the film's end hard to achieve. As it stands the ending is still a bit ambiguous and I wouldn't be at all surprised if Ultron is brought back to help fight Thanos in the Infinity War.
Age of Ultron is almost overstuffed with characters. This isn't much of a problem in the books since comics writers have decades of experience dealing with large numbers, in fact that might be a unique writing experience since maybe only soap operas even approach the same situation. Some of the techniques used are rotating characters, breaking them off into smaller teams, focusing stories on subgroups within the larger one, etc. But a movie is locked into a cast of actors and expectations from the previous film. All have to get a certain amount of screen time and things to do but then again this works against showing a larger picture - this same story in comics could easily show dozens of other characters fighting Ultron across the globe in just a few panels.
Which brings us to the Hulk problem. Why is he here? Clearly it's because he was in the first film but in story terms there's no purpose. In the first film Fury manipulated him just as Fury manipulated all of the Avengers and in the end the Hulk provided something of a wild card status and needed combat heft. In the first sequence of Age of Ultron he's out of place in what's basically a military operation. (And speaking of that it's clear why the others kept their costumes but Hawkeye and the Widow really should have been wearing white camo.) The Hulk's next appearance shows how much of a liability he is - more importantly the Avengers are aware of that and prepared Hulkbuster armor (called Veronica in the film for an oblique reason - Banner dated a Betty and in the Archieverse Betty's nemesis is often.....)
Another comic book element that's a bit grating in the film is all the tosses to upcoming movies, at least four (Black Panther, Captain America: Civil War, Thor 3 and the Infinity War). Comics are often narratively porous, referencing not just a current storyline but referring to numerous other series, older events, classic images and even elements from other companies. (Just think of the government hit team based on the Avengers who appear in The Authority.) This web of references don't work as well on film and here only the Black Panther bits seem workable because most viewers won't realize that's what they are. But Thor heading off for a mystical bath that's conveniently driving distance from a college?
The Scarlet Witch's powers are still about as vague as in the books but she comes across fairly effectively (and even has a "look" towards the Vision). Dream sequences and mental visualizations such as showing a character's fears are not only cheap writing tricks but rarely even a minimally effective one. Here it seems less than useless. So Stark is afraid of his friends dying and having survivor's guilt? We need a fear trip to reveal that? Thor is apparently afraid of parties and Captain America of swing music. I think we're supposed to read this as something Thor deals with in his next film but it really just seems a reason to give him screen time. And apparently Steve misses Agent Carter though again we already knew that. Natasha on the other hand seems to have memories more than fears though this isn't made particularly clear and none of it shows how brutal her training was like the talk she has with Banner.
Why even bother killing Quicksilver? He has the fewest lines and least personality of all the main characters so I doubt many people cared. Which clearly is also a reason - the film shows the fight as having consequences but not that bad. (Unlike the Daredevil show which had a genuinely unexpected death.) If Whedon really wanted to make an impact it should have been Hawkeye but that gets to the point above that many creative decisions in a film like this are made for contractual and business reasons. (Though on the other hand comics at times kill off characters willy nilly confident that they can be brought back when needed - we can guess the movie Civil War won't end with Captain America dying because we know he's in the next Avengers films.)
And by the way, Captain America's little speech about innocent people dying every time - every time - somebody tries to stop a war before it starts is quite rousing. But stop a minute. Cap is telling us to never try to prevent wars. That's a little harsh and short-sighted even for a professional soldier.