Fair question. After all, the populist voters on Internet Movie Data Base have rated it the 29th best film in the 120-year history of world cinema, last time I looked. They're thrilled because it met every one of their expectations for that genre. I was hugely entertained but not thrilled, because it didn't exceed any of mine.
To be great, a work of art has to take you somewhere you haven't been before. Maybe a chef introduces you to new combinations of flavors. Maybe a painter makes you see a common household object in a new way, as Cezanne does with his miraculous pears and apples. Maybe a composer, bound as they all are by the dozen notes of the musical scale, arranges those 12 in a way that creates new beauty or drama.
Sometimes a filmmaker pulls familiar pieces into a new pattern, as "Avengers" writer-director Joss Whedon did with Drew Goddard in their script for "The Cabin in the Woods." But if you start out with a template and do all you can to stick to it, element by element, greatness remains beyond you. Nobody ever became a master by coloring carefully within the lines.
Schoolteachers have ratings of "meets expectations" and "exceeds expectations." Sometimes a student wonders, after doing work better than anyone else in the class, why he didn't get the higher rating. A likely answer: He didn't live up to his potential, which was also higher than anyone else's in the class. He settled for the best that others could do, not the best HE could do. And "The Avengers" isn't the best Joss Whedon can do.