I'd say character development, which is also a criticism that's being levied at MOS for lack of it. Trust me i'm no huge Avengers fan but i can see what most people liked in it, critics and audience.
Even on the character development front the Avengers had massive issues. You pretty much had Bruce Banner and Tony Stark, with everyone else being a sideshow. While I do find most of Snyder's action sequences to be tedious at times, the same thing I can say about the Avengers- mostly because there is no suspense; you pretty much expect everyone to get out in one piece.