What defines the art-style of Japanese manga? Even though manga has lots of different art styles we still have an idea that certain styles are "manga style" and other ones aren't. What is it about an art style that lets us know it's manga style? What elements reappear across many manga? What distinguishes our idea of manga style from our idea of Western comics style?
I'm not sure there is a specific answer to this. As you acknowledge, different artists have different styles. Both Japanese and western books vary from "cartoony" to "realistic". There are cultural differences seen in the art, such as in Japanese media adults often smoke, while in American media, they seldom do -- definitely not in titles geared to children. (Maybe they do in French media; that's why I switched from "western" to "American".)

Hell, sometimes an artist has extremes in his own work; consider Yoshiro Togashi and his "bad days" on Hunter x Hunter.  Here is a comparison of a page that got published in the weekly mag, versus how he improved it for the HxH volume:



Is the left even identifiable as manga vs comic book (vs fan art)? On the right, the main clue as to being a manga, at least to me, is the eyes of the upper right character. Otherwise it could be from a (somewhat dark) American funny-animal comic book, a la "Fritz the Cat visits a jungle".

Let's look at another page:



Is there anything innately Japanese or western concerning Jack Katz' work on The First Kingdom? If I didn't already know, I could not tell anything except that he is a remarkable artist (a mangaka in every sense of the word except his nationality). ... Hmm, maybe in Japan they would have to fade-out all the nipples.

I myself instantly recognize manga because the pages have to be read from right to left. The other instantly recognizable hint is that manga are typically b&amp;w, whereas western comics are usually color.