There be some minor spoilers
1) Vision. Pretty well executed, has some of the more insightful lines. Does one awesome "surprise", which most people wanting to see it will have heard about in a day or so. Ties in well with the broader plots of the MCU. The one element I liked best there is that for a universe with so much technology, it's one of the hallmarks to say "technology isn't necessarily going to kill us all". Even with Ultron around. There's a question there about how we use technology or how we allow it to evolve and what it can do for us, or to us.
2) Hawkeye, Hulk, Black Widow all seem to have good plot arcs/threads. It's basically a Hawkeye/BW movie really. This is good because those two are probably the best two actors in the franchise and seem like they actually enjoy doing it (along with RDJ), as opposed to some of the other skilled actors/actresses that don't (Hopkins/Portman seemed really bored in both Thor movies). Thor also gets to do more it seemed like.
3) A few good lines still to make for humor (Vision, Hawkeye, and Thor have some of the better ones. Iron Man isn't so much). Some weren't landing or went on too long. Stark-Ultron dynamic sometimes worked and sometimes didn't. The fact they're still trying to be funny or sarcastic or witty once in a while is a critical break from the "no jokes" world of DC. Daredevil still had some cynical/sarcastic humor despite being way darker than this was. This was also a little darker as it was, exploring the dark side of (most of) the characters. So the humor helps, but they could have played it more like Winter Soldier or the first Avengers film.
4) Scarlet Witch has some very cool moves. Ultron has a line about how she will tear them (the Avengers) apart, and she lives up on the damage she can dish out.
5) War Machine and Falcon cameos. As sidekick characters go, they've done pretty well casting some of these. I'm sort of curious if they cut some of Falcon's scenes or not.
6) Klaw's introduction was very good also. I'm more curious how Black Panther will go as a result.
1) Quicksilver was much cooler in the mid-sequence in X-Men. Expectations being that high, they didn't do much with his speed and basically gave him one line (which was not "I am Groot"), and a monologue to pass that off as character development. His sister does a little more movement.
2) I'm not sure they worked out the twins' plot line that well. They went a little too smoothly to the "enemy of my enemy is my friend" logic. I realize Captain America is supposed to be really charismatic and awesome, but he's basically the only reason they do as far as I could tell. They had a pretty compelling reason to be pissed off in the beginning.
3) Doesn't really have a firm examination of the ethics of AIs. There's a nod toward the idea that they won't necessarily be evil, but that's as far as it gets it seems like. Vision is good because...? I'm not sure where they established what makes him that way. It's boldly stated, and then he does good/heroic things in the show/not tell mantra that Marvel succeeds at where DC has been failing for a couple movies. But what actually made him different than Ultron? We get a glimpse of it at the end, but it doesn't really proceed during.
3a) Ultron basically feels like they made a Dr Doom as villain movie plot line without embracing who Ultron was at times. The trailers made him out to be a pretty creepy robot (quoting Pinocchio for example while engaged as a fanatical murder robot), but he did not go much farther. There's some nods toward why he is the way he is being crucial aspects of his creation and design. This also means the villain isn't very interesting. Or at least feels like he could have been much more interesting. He really quickly goes off the rails and we don't really get why for a while other than "audience expects an intelligent computer to be homicidal". They've been struggling on the villains other than Loki so far.
4) Most of the fights were messy/choppy. The first one right off the bat was pretty smoothly executed, and did a fair amount of showing how they've worked as a team since some of the improvised cooperation in the Battle of NY in Avengers 1. Later fights felt choppier. The Hulk-Hulkbuster fight wasn't too bad, but the Cap-Ultron one was rougher, more like a Batman fight. The fights in the freighter also were meh. Daredevil and Winter Soldier may have ruined that expectation on how the fighting can go in a superhero movie.
5) Whedon has a pretty firm dialogue stamp. Which occasionally means that some of the characters feel off from previous movies (that he wasn't involved). Captain America in particular comes off much differently than he did in Winter Soldier, and also Thor.
6) I'm guessing some of the stuff on the cutting room floor would have helped unpack some of the jumbles on character development or a couple plot arcs (Thor's is kind of abbreviated at times). Supposedly the DVD release will have all this cut stuff in it.
On balance, it's still a very entertaining film. It's popcorn stuff, if you like popcorn. But it's not much more than that. Winter Soldier and to some extent Daredevil have these broader social nods to things actually going on (drone warfare, surveillance states, corruption/brutality by police, etc) that make them feel a little meatier than a standard comic book film. This doesn't so much. It's basically Guardians of the Galaxy but with less music and more Avengers.