There are plenty of great comic book young readers such as Tintin or Bone, but it seems that American comics have
been strictly targeted towards children for so long that comics for adults go out of their way to be transgressive in a way that would scare away or at the very least disinterest a non-comics reader. There's also the factor that that too many comics rely on the superhero genre, and the non-superhero comics are often not commercial enough to appeal to a large audience.
Watchmen
-Probably one of the darkest works of popular fiction, filled with nerotic, repulsive characters and violence
Dark Knight Returns
-Violent and cynical
Sandman
-Opening arc easily the weakest part of the series
Moore's Swamp Thing
-The beginning heavily revolves around dangling plot threads from Martin Pasko's run
Love and Rockets
-Gratuitous sex
Black Hole
-Frank and graphic depiction of adolescent sexuality
Most of Adrian Tomine, Chris Ware, and Daniel Clowes' comics
-Depressing, cynical, and/or nihilistic
Saga
-Pornographic levels of sex
Claremont's X Men
-Infamously dense
Stan Lee's Marvel Work
-Like the majority of comics from the 30s to the 80s, has a style of writing that is considered outdated by today's standards.