I think the best way to describe Age of Ultron is a lot of "great" moments that come together to simply make a "good" movie.
There's a lot to love, blatant fanservice aside (Hulkbuster armor, Warmachine/Falcon cameo etc.) there are probably several resonating scenes within the film that you'll absolutely adore. However, when all these little things come together you're almost left with a feeling of dissatisfaction.
Marvel is evolving filmmaking and Age of Ultron isn't really a movie in the traditional sense, it's an episode. It spends roughly 35% of screentime setting up a healthy future for the Marvel franchise from the Infinity Gauntlet, to Civil War and the Avengers initiative to Thor's Ragnarok etc. there's a plethora of nuance that hints at world building material that hinders the quality of a standalone film.
And that I'm actually pretty cool with! It's just that people expecting a movie in what we've come to understand them as may be taken back by this particular approach of filmmaking. For example we aren't given ample time to breath anymore, ever, with the incredibly large cast we're quickly dictated to feel a certain way about a character almost immediately. Relationships rapidly develop and fall, (Natasha x Bruce) identifying backgrounds come out of nowhere (Clint the family man), likeability must be established so we feel something upon their passing (Quicksilver), and it's not that any of it doesn't make sense but directors are going to have to start being careful at the hasty rate they present us all these "mini-stories" or else we'll choke before we can swallow any of them.
Due to this relentless velocity, a lot of the movie does in fact come off contrive. Like cleverly using Wanda's powers to give character development through flashbacks or abruptly revealing Clint the everyday man because a team of "gods" needs someone down-to-earth to keep them relatable or creating a "much needed" romance out of a plot device required to control the Hulk. These are a few examples but you get the idea, when convenience is seen as a writing tool it's quickly interpreted as a sham.
The plot isn't as intelligent as I had hoped, themes of global instability due to the Avengers dominance as a potential military force that were touched upon in "Winter Soldier" wasn't revisited nor were politics like backlash from other countries due to collateral damage from the Hulkbuster vs Hulk fight. Where has the relevant social commentary that made the original "Iron Man" such a hit gone to?
Another issue I personally had with the film was Ultron's portrayal, his familiarity and constant use of humor, an incredibly human trait, was really off-putting! He's not the enigmatic, cold, omnipotent robotic being he was meant to be. There were a lot of stellar moments for the character though, like the scene where he accidentally rips the man's arm off like a child feeling anger for the first time, forgetting the fragility of the human body. It was perfect! Or his final conversation with Vision before his death, also a really well written point that I truly enjoyed.
I also enjoyed Natasha's past with the Red Room and her coming together with Bruce as a couple, Ironman and Warmachine blasting shit out of the sky, Hawkeye helping Wanda overcome her fears, Steve telling Tony how he had come to terms with his past and found his new home...
There really is a never ending list of wonderful moments in Age of Ultron but that's what the movie is. A lot of commendable individual scenes but when they come together to be enjoyed as a coherent story it simply stands as "good" entertainment. Again though, that's not so bad if you accept the fact that Marvel is evolving what defines a movie. They're now episodes of something much larger, a constant growing world that should be viewed subsequently of each other.
It may not have the novelty or grandeur of the original film, but what Age of Ultron tells us is that Avengers movies will always be above-par entertainment. Never forgetting the importance of blending action with emotion and character. They also tell us "Hey you didn't love this movie? Fine we have a lot more in store for you in the future that you'll probably like anyway"
Which y'know, is fucking smart money-making. $__$