I disagree about Guardians of the Galaxy -- I'd put that as one of my favorite Marvel movies, maybe even #1, and Age of Ultron perhaps below one of the Captain America movies. I'm guessing this comes down to personal taste and some people may feel strongly one way or another.
Overall: I enjoyed the movie, but my faith in Marvel has been shaken a tiny bit. The characters are still fun, but things felt disjointed. Ultron was unexpectedly one of the most entertaining parts.
In particular, I kept feeling like lines and scenes in the first half were oddly clipped and edited. I can't remember which scene, but I think one of the Avengers was talking to a woman, and it was some sort of important or emotional moment, and the scene ended almost mid-sentence with a cut to black or another scene.
Even had the editing been smoother, I think this movie may have been a bunch of flashy scenes with Avengers doing flashy things, rather than a bunch of scenes that fit together into a story. One might say that of many Marvel movies, but I do not remember it being true to this extent. Granted, I still LIKE the various characters, and they have their chemistry, and they are fun to watch though Thor's accent felt a little forced or sloppy this time.
Again maybe this was true of previous Marvel and Avengers movies, but a couple of moments felt particularly contrived. There were also some slow, emotional moments that kind of clashed with the frantic pace of the rest of the movie instead of pleasantly contrasting. Hawkeye outright tells another character that nothing really makes sense, but the movie still tries to keep things sort of grounded instead of completely rolling with it.
There were a good many jokes, and even with a reasonable crowd late on opening night, some lines didn't seem to get a reaction. I had trouble even understanding some dialogue -- someone fake-coughing a joke, or someone yelling a joke in a fight and being overwhelmed by noise, that sort of thing.
On to the positive parts, I might have expected Ultron to be dramatic and dry like Bane. Instead he turned out to be randomly wisecracking and that was fun. Black Widow showed a bit of emotion and character depth instead of just being a pretty face. There was at least one case where I did not expect what happened in the plot. I thought Scarlet Witch's actress worked well, and the not-magic rendering was cool. Seeing the Avengers together doing things, and hearing the Avengers theme, were both fun.
Sony told the truth and there was no post-credits scene, only a mid-credits scene.
That may sound like a lot of negatives and not many positives, but I still enjoyed it for what it was. More comments are spoilered.
Someone suggested the coughing joke was "testosterone", when Tony Stark and Thor were trying to outdo each other at the party.
Black Widow's history was touching. Did I understand the implication correctly and her ovaries were completely removed to try to make her more of a heartless killing machine?
I think the Hawkeye misdirection was actually pretty well done. They set everything up so you feel bad for him and he's all prepared to die dramatically. I didn't expect Quicksilver to be introduced and suddenly killed in the same movie. He was a cool guy.
Ultron was weirdly religious with the church symbolism, the mention of God and Noah, and so forth.
I had probably heard the name "Vision" mentioned but didn't know much else. I was pretty surprised by that character. I instantly thought they looked like a member of the Blue Man Group, but in red. My theater seemed to like them lifting the hammer, which was a big deal. Having an otherworldly android-esque character who was somehow moral, worthy, and/or "good" contrasted well with Ultron.
Two of the most contrived moments I mentioned earlier were Black Widow's captivity and Thor's bath.
Black Widow falls out of the sky but somehow survives, presumably caught by an Ultron drone. She then gets threatened by Ultron who wants to show off...by destroying one of his suits with a better suit, and then locking her in a cell. Ultron then apparently leaves the cell and the entire facility unguarded for Banner to sneak in and shoot the door open. Maybe he shot a robot or two with a laser, but Banner isn't usually known for using a gun well.
Thor's case in particular seemed abrupt. Thor needs answers about his dream. He talks to the professor from a Thor movie, who happens to know about a magical pool, which happens to let you revisit visions. Thor jumps in it, sees a different part of his vision, and jumps back to the action. It was funny seeing Thor in a hoodie, but the section felt a bit shoehorned to me.
One thing I didn't fully understand was Scarlet Witch and Quicksilver's motivation. Their parents were killed by a bomb from whom? In what war? Would Tony Stark have even been old enough to have been involved with the weapons for that war? I guess he could have been in his 20's and already in the company business.
This makes some sense but maybe part was cute from the movie about Ultron:I don't think Tony's brain waves were even hinted at, so I'm not aware of an in-movie explanation as to why Ultron has part of Tony's personality. The current cut of the movie just seems to show Tony and Bruce attempting to interface with existing software.
Natasha hasn't been "just a pretty face" since the Avengers so that's a weird thing to suddenly comment on.