I think Ironman 3 is what sparked the "humor debate" in these MCU films. If you look at the trailers, Marvel blatantly misdirected people into thinking we were getting a darker, edgier Ironman film that was going to deconstruct Tony Stark, and I think most of us were all for a movie like that. Mandarin twist aside, I think a lot of fans, including myself felt like the wool was pulled over them with the overabundance of not only humor but deflation of tension. Every time something cool or badass occurred it was punctuated with a joke or a "gotcha!" that took away from the moment. I think it was from this point where Marvel started to take their humor a little too far.
The phase one movies balanced humor and serious tension so well, and then in Phase two you could really feel the Disney influence (besides Winter Soldier). Humor is a good thing and I think its essential to have levity in these movies but I get annoyed when the tension is deflated when a serious scene or moment is punctuated with a joke. AoU would have been a much better film is Ultron wasn't so quippy and had more menace in his scenes.
At the end of the day, however, Marvel and Disney are playing to their audiences, and those of us upset at this overuse of humor are just a vocal minority.