JUST GENERAL SPOILERS I SUPPOSE:
Ultron wasn't scary at all which was its worse sin. James Spader's voice lends itself to being so sinister and dangerous that him making jokes all the time put me off. To the point where when Hulk jumps of the jet at the end and he exclaims "Oh for Gods sake" I actually groaned. He never actually seemed like a physical threat to any of them, when he fought Iron Man and Thor they just kind of twiddled in the air for a while.
Hulk and Black Widow was just weird. It never crossed my mind that The Avengers were actually in danger. The ending with the helicarrier turning up was beyond stupid. Everytime something cool was about to happen they cut away "Oh, look! Thor is about to go toe to toe with Ultron I guess we should cut away to ten minutes of Captain America herding civilians!".
Oh also the end of the Hulkbuster was really stupid. You have established that The Hulk is basically an unstoppable force, it seems like they just needed to end the fight quickly after they couldn't think of any more spectacle to throw at it. It takes one little random punch at the end to somehow stop him?
I know people say that DC is trying to go too serious but I think that this film was way too Whedon, too quipy for its own good. Love it or hate it atleast Man Of Steel mad a splash, every Marvel movie at this point in my mind I just come away from it think "well that happened". All they seem to serve is building Avengers 3 & 4 at the expense of their own story.
Also the ending where they are all buddy buddy completely threw me off. Maybe my expectations were off but I was really hoping this movie would kick off the Civil War in a big way, where the team barely survives Ultron and at the end are in complete disarray. Also for what happened Tony Stark should really be given a kick in the balls. The man almost destroyed the world and no one seems to mention this.
I did like how Hawkeye and the twins were played up though, was unsure about Quicksilver and Scarlet Witch before going in but I thoroughly enjoyed both, and was pretty ballsy to kill off Quicksilver at the end instead of just hanging onto him for the sake of it.