Every so often someone out in the comics interwebblogosphere takes it upon themselves to compare "American" comics to manga, generally with the intent of showing why people prefer manga to "American" comics (usually: cheaper per page, longer, greater variety of subject matter). Almost invariably, when they say "American" comics they really mean "direct market super-hero comics" and they completely miss the wide variety of formats, lengths and subject matter of the broad range of comics in the Americas.
On the one hand, it's easy to see how one might be led to believe that American comics = Super-hero comics; if one walks into many comic book specialty stores in this county, they're likely to see super-hero comics dominating the displays.
But it doesn't take that much effort to venture out to see what other American comics are readily available. Now we can't expect everyone to be familiar with the works of Kevin Huizenga or Ariel Schrag or Jason Shiga or Carla Speed McNeil. But the following list of comics from the Americas aren't obscure, low-circulation comics; they're comics that have received plenty of press, been reviewed in non-comics-centric publications, and can be found at any well-stocked bookstore (and many have been turned into movies):
Dan Clowes, Eightball etc.
Dave Sim, Cerebus
Hervey Pekar, American Splendor
Neil Gaiman, Sandman
Frank Miller, Sin City & 300
Phil & Kaja Foglio, Girl Genius
Larry Gonick, The Cartoon History of the Universe
R. Crumb, works
Jeff Smith, Bone
Terry Moore, Strangers in Paradise
Alison Bechdel, Fun Home
Will Eisner, works
Steve Niles & Ben Templesmith, 30 Days of Night
Sergio Aragonés, Groo
Los Bros. Hernandez, Love & Rockets
Chris Ware, Acme Novelty Library
Craig Thompson, Blankets
Greg Rucka & Steve Lieber, Whiteout
Jim Ottaviani, works
Bill Waterson, Calvin & Hobbes
What you'll notice about the above works is that they cover a wide variety of subject matter: humor, horror, dark fantasy, memoir, non-fiction, drama, thrillers, steampunk, historical epic. And they come in a wide variety of formats: some are published first as floppies; others are OGNs, some begin their lives as Webcomics.
I'm not suggesting that everyone should have read all of these comics, but you should at least be aware of their existence before writing about "American" comics.