Same here. It's the colors - the 80's colors really killed it for me. I've seen some of the black and white pages and they're much easier on the eyes. XD
Graphic novel is a term coined by people who generally consider comics to be low-brow. So, comics like say, The Tick will always be comics, but "high brow" comics - comics considered worthy of the literature community at large - such as The Watchmen, or 300, or Bone, are granted the glorious title of "graphic novel". We had a very long discussion on this in my creative graphic novel class. XD
There's no difference between a comic and a graphic novel.
On top of that most manga is in the "graphic novel" section of a bookstore - which, by those definitions, means ALL hard core manga fans read "graphic novels". XD
Er, it's read right to left.
I think the key differences have little to do with art style - that will always come down to opinion. It's the mechanics that really set the two apart. Panel layout, pacing and design are oftentimes miles apart from your standard American fare. Japanese comics are usually (but not always) less wordy, with more emphasis on facial expressions and action. This ultimately effects the pacing; manga is meant to be read quickly, so there will be less to read in the same ammount of pages as an American comic, but about the same ammount of visual information - if not more.
Take the Watchmen for example - had it been drawn with Japanese pacing it probably would have been two - possibly even three times as long simply because the "wall of text" speech bubbles would be broken up and distributed over several panels - if not several pages. Plus, there would have been fewer repeditive pannels - IE the pages and pages of identical rectangular panels. There would have been diagonals, more longshots, more close ups and - what I prayed for while I was reading it - more silent panels.