It all changed for superhero movies in 1989 when Batman premiered in movie theaters. The Tim Burton film is widely regarded as the one that paved the way for the mature (and perhaps slightly dark) modern superhero flick, and if you watch it, you can see why.
From its opening scene, you can feel Gotham City alive with the crime and corruption you had previously read about in the comics. It's eerie and yet classy art-deco inspired designs doubled down on the nightmarishness, while the grim shots of alleyways also grounded it with some realism. In many ways, Batman looked like a crime noir movie, and it's a look that it wore well. From Grissom's meeting with his mob to the smoky alleys Jack Napier had his dealings with Detective Eckhardt, the genre was at home in this style.
Speaking of Mr. Napier, Jack Nicholson was a force as The Joker, shifting from his typically clam and collected approach for the mobster and a ridiculously fun over-the-top performance as his clownish alter-ego. The subtlety of Michael Keaton's Batman is the perfect foil for him, as the movie brings these two larger-than-life characters to life in such compelling fashion that they became the definitive portrayals of the characters for so many.
Batman is a collection of genres that turned the superhero genre on its head, showing it that could be more than one thing and that it had more mature stories to tell. It's serious, outlandish, artsy, and grounded, and so much more all at once. It's a game-changer, and the superhero movie owes it a huge debt of gratitude for everything it continues to accomplish.
It is, without a doubt, one of the greatest feats in movie history.