Everyone loves superheros. That's why a superhero movie will never win an Oscar--it's too obvious. The genius of Birdman is that it incorporates a superhero theme into a complex world rich with intellectual pretension.
Anyone who takes art seriously adores intellectual pretension. This is the stuff that wins Oscars. The problem with it, however, is that it can fail as entertainment. So: add superheros.
Did Birdman win just because of the subject matter? This sounds ridiculous, but it might be a fair observation. Obviously it won the Oscar in part because it was well executed. But let's be honest: it's a given that the script was well executed. Look who wrote it: very highly skilled experts in the craft: Alejandro G. Iñárritu and company. But these people aren't any better, necessarily than, say, Wes Anderson...
If you are among the elite in terms of screenwriting skills, this should be your primary goal: to combine the pop with the intellectually satisfying. It creates something huge. In music, it's called an anthem, and it's what Bob Dylan created again and again with his hyper-poetic yet amazingly relatable songs: Like a Rolling Stone, All Along the Watchtower, Blowing in the Wind, Mr. Tambourine Man, etc., etc. If you can do this--if you can create anthem after anthem--then you become Bob Dylan...or Francis Ford Coppola...or Birdman: a living legend.