As I expected/feared it was Avengers 1 with a fresh coat of paint. Fortunately it wasn't as tired or bad as I expected. In fact i'd say it was better than Avengers 1.
Villain: Age of Ultron. Much better solo physical threat. Had a much better plan. Were it not for the obligatory plot trapdoor that gives the heroes a way out he would have succeeded. Loki's plans of conquering Earth with the Chitauri were finished the second Thor showed up. Loki was a better character, however, although both of their motivations were kind of generic. Ultron's only flaw was that
Spoiler:
he didn't want to kill the Avengers right off the bat
, blame his ego.
Plot: Age of Ultron. Not much better than the original, and both are pretty thin, but at least the heroes needed to do some detective work in order to figure out Ultron's plans whereas Thor told everyone about the Chitauri right of the bat. It also did an adequate job of introducing a few new significant characters.
Musical Score: Tie. Neither has a remarkable soundtrack. A slight nod to the original for blasting the main theme perfectly during the "I'm always angry" scene. AoU missed that.
Dialogue: Avengers. Not necessarily better writing, but the subject matter of the protagonists coming together for the first time led the better conversations. AoU also went overboard with the jokes.
Characters: Tie. A smaller cast should theoretically lead to more focus on each character but the only person the original cared about spotlighting was Iron Man. AoU at least has an excuse for spreading it out but still managed to focus on Hawkeye. Speaking of which, why is Hawkeye labeled useless and not Widow? He has a ton of specialty arrows and martial arts skill, all she has are a couple of pistols and hand tasers with martial arts skill with considerably less muscle mass. I guess she's better eye candy for public relations but come on. Also
Spoiler:
the impact of killing off an Avenger (who they should still be able to bring back Coulson-style) was lessened by the fact that it was his first film.
Action: Age of Ultron. Quality and quantity, hands down.
I won't deny that there is this strange, underlying hollowness to the film, but it's still second in the MCU to me, behind The Winter soldier.