Whedon might have made it look deceptively easy, but he still had to juggle a LOT of different story elements to make the thing work. He not only did that brilliantly, but in the end made an even better movie than most of the ones that led up to it, which is no small feat.
Whedon was able to juggle all the characters and make the movie actually about the TEAM.
Everyone got some development and a chance to shine. It wasn't all about Iron Man like I feared it would become.
Those X-Men movies focused almost entirely on Wolverine while the rest of this teammates were shoved to the background. I still won't forgive them for what they did to Cyclops.