The key point to the superhero movies is that since they draw from comic books, the plotlines are at least half-decent. The effects are certainly good enough that they don't have to worry about them, and since the characters are generally well developed, they just work well.
It's the movies like Transformers or San Andreas that are the "bad" popcorn flicks, where they don't really bother with plot or characters, they basically just want things to blow up. And then they shoehorn in a plot or characters after the fact, which really doesn't work.
To me, the academy type movies are your steak, the superhero movies are a good burger, and the ones like Transformers are like a McD's burger. So certainly I wouldn't put the Avengers up with the best movies of all-time, but I damn like my hamburgers, so if I haven't had one in a while, it tastes great. The big problem is that we're starting to get over-saturated, especially once you throw in the TV shows like Daredevil (or I guess Arrow and Flash as well) into the mix, but at the same time, after every Avengers movie I hate it when I look and see the next one isn't out for another year or two.