So for the most part it felt like this film should've really been called "Avengers: Civil War-Prologue" since the majority of the fims narrative was just to set up Civil War, as well as Black Panther and the next Thor film. So it never had the feeling of the epic culmination of previous films like the first Avengers had, and it was more of a stepping stone. The humour in this one fell flat for the most part for me as well. There just seemed something off about it but couldn't put my finger on it. Like almost every joke was poorly timed. One joke about Captain America and swearing, which wasn't funny the first time, was repeated about 5 or 6 times throughout. The action scenes were obviously great, but the best bits were saved til the end when they actually had to rescue the civilians rather than just meaninglessly pound on robots. There were a few great group shots like the spinning shot from Avengers. One right at the beginning with the 6 core guys, one in the final battle with Vision and the twins added in, and one at the very end with the New Avengers which was great. The party scene from the trailer was very good just because it was cool seeing all these characters just interacting socially. Speaking of the characters themselves...
Banner and Black Widow - Im not really into the idea of these two hooking up and felt the only thing it added really was the emotional spark at the end when Hulk left the group.
Thor - Didn't really do much other than add some muscle to the group and then set things up for Ragnarok. Moved the infinity gems arc along though. Constant running jokes about Mjolnir which were hit and miss, the best coming at the end along the lines of "so if the hammer's in an elevator, and lifts it, is the elevator worthy?"
Cap - Again was mostly used to set up the next film and add a bit of muscle to the group
Iron Man - Was front and centre again but seemed to lack the usual wit, and again just setting up Civil War with him and Cap going at each other.
Vision - Straight out of the comics. A bit rushed, but fine.
The twins - Wanda was great, Quicksilver was okay. Fully expected his death since every other actor had already been confirmed for future films.
Ultron - I still genuinely don't know what to think about Ultron. His creation was really rushed, but the introduction was cool, much of which was seen in the trailer. I didn't like how he was so full of mirth and wisecracking all the time though. Was hoping his character in particular was going to be much darker than it was. The Crimson Cowl nod was good though.
And finally Hawkeye - Absolutely stole the show. The "average Joe who grounds the team and keeps this gang of powerhouses together" gimmick worked to perfection, pretty similar to how Whedon used to use Xander back in Buffy. He was the funniest character too which is a huge credit to Renner's delivery which for some reason seemed a little Bill Murray-esque to me. "We're on a floating island, fighting an army of robots and I'm here with a bow and arrow. None of this makes sense." I liked how he was the only one who could beat Wanda too.
Post credits scene with Thanos was sweet but nothing new really.
Probably sounds like I didn't enjoy it, but I did. Its just that the faults are fresher in my mind because I was expecting something more.