Batman is one of the most well-known super heroes of all time. Originally appearing in Detective Comics #27, Batman soon became one of the cornerstones of DC comics.
His original outfit design:
The hyper-bright yellows and greens and oranges are gone. Dark grays, washed-out blues, and occasional yellow accents became the norm in Batman comics (and still are, in most Batman comic book lines). The comics move more towards heavy plotlines and ideas, and display the grisly aftermath of Batman's decisions in vivid detail.
As a no-longer-kid, I can see the advantages of the grim, dark, subdued art style. Kids like the lighter, funnier Batman because he is THEM in their imaginations. They imagine themselves kicking bad guys and tying them up with Bat-rope and leaving them for Bat-mom to collect as she cooks dinner.
But adults have a darker view of life, and it is displayed in their comics. For an adult to associate with Batman, the hero must have the same inner struggles as the reader, and must follow the same "laws of the world" that the reader follows.
That said, I miss the times when the bright pastel Batman and Robin would entertain my imagination for hours.