I'll try to keep this spoiler free.
Black Widow seems to be whatever she needs to be. We get a bit of back story, but her personality, the way she interacts with others... I don't think she lets people in. Ever. But I think that works for her character. And there's some great interaction between her and Banner. Also, how tough must that scene with Banner when she's talking about her graduation ceremony have been given Scarlett's condition during filming?
Iron Man... is a man who has a vision, and it's a terrible one at that. That vision drives everything he does. And while he's a dick about it, he's the same Iron Man we've come to enjoy watching.
I'm really enjoying watching Captain America grow as a character. It's subtle, but it's there.
The Hulk is incredible. But really that's all Mark Ruffalo. He's seriously such a solid actor... and he brings such humanity to the monster. His story arc though AoU is clear, and well played, and makes perfect sense for the character.
The bit with Thor's hammer is funny, and it comes back swinging later in the movie. Thor himself gets some screen time, but from what I've been reading and hearing, a bunch got cut. Hopefully the DVD release will add it in. He does reveal some knowledge about the infinity stones that ties things together.
The Twins, Scarlet Witch and Quicksilver, really well done. She's creepy, he's kind of cocky. I feel like I can't talk too much about them without spoiling it.
Ultron is clearly a program of Tony Stark's creation, which is the only reason I think the quippy one-liners and flawed motivation work as well as they do. I think the mouth animation was unnecessary, but otherwise liked the look. And James Spader as a villain is just delightful.
Hawkeye I definitely can't talk much about. He had the one OMGWTFBBQ moment of the film for me. He also got a serious amount of screen time, and one of the best lines in the movie, “We’re fighting an army of robots, and I have a bow and arrow. None of this makes sense.”
And finally Paul Bettany's Vision. I'm actually not very familiar with Vision as a comic character, but Paul's Jarvis has been with us since the first Iron Man movie, and he's another one of the great actors to join the MCU.
In short? I thought it was solid, if not quite great, but definitely worth seeing, and maybe seeing again.