First, I musty say- don't believe the naysayers. This movie is a blast, a ton of fun. Whether its better than the first Avengers (its DARN close) is debatable. Is it the best Marvel movie- no, that title belongs top Cap 2. The truth is Avengers 2 is an action packed movie with a good sense of humor.
I think the problem is expectations. After Avengers 1 and Cap 2, the expectations were sky high. Through the roof. Impossible to reach. I suspect that some who haven't enjoyed this movie simply kept comparing it to Avengers 1- and their DREAMS of Avengers 2 should be. It was never going to match that.
I must admit, the movie has flaws. It feels rushed in several areas- particularly with Ultron himself (who is fascinating but needed more time to develop). There were some beats that didn't quite work (Quicksilver). The movie also feels overstuffed with characters- they ALMOST manage to successfully juggle them, but they don't quite get it right (unlike Cap 2- when you think about it, that film has lots of major and minor characters, but they balance them very well). They dispense with some things far too quickly (von Strucker).
On the other hand, the film gets a ton right- more than enough to make up for its flaws. The team's chemistry remains spot on, and each of the Avengers gets a chance to shine at some point. The humor is also very sharp and witty. The action scenes are top notch- filled with excitement but never too much to follow. The movie (despite its 2 1/2 hour length) moves briskly. I was also very happy with how they handled Vision. I can't wait to see more of him. The movie also has a bigger scope, with the Avengers running to several nations to fight Ultron (Korea, Eastern Europe, "Wakanda"). Ultron was an impressive villain- a dark reflection of both Iron Man and Banner (I just wished we got more time with him).
So- I didn't give any spoilers, but I think Avengers 2 is a great movie- action, humor, good actors. Better than Avengers 1? Does it have to be? I don't know. I do know it was a fun time at the theater, and it certainly is strong enough to be in the Marvel Pantheon. For me, the Marvel films are fun, and I love watching them. But- they are not deep films. Certainly Nolan's Batman is deeper and more thought provoking- taking current issues and using them for comic book stories while also having emotional resonance. The only Marvel film that has done that is Cap 2- NSA / drone strikes along with the sinking feeling that Cap's fight against Fascism / Totalitarianism has not been won, rather it is only just begun. Its current, but also resonant emotionally. The rest of the Marvel films are fun affairs, and seeing these characters handled with such respect has been a pleasure for this former comic book collector.
If you like the Marvel films, go see Avengers 2. It is, at the very least, a fine comic book film. Chances are you will enjoy the heck out of it! I know I did. In fact, I am hoping to see it again soon...