My general take on the issue is that comics critics are writers and writers (a) tend to pay more attention to and (b) are more comfortable opining on "writerly" stuff - plotting, theme, character, etc.
The same thing happens with film critics and acting: most will say whether a performance is good or bad, but few go into any detail about what actors actually do.
Tim's take on the issue is that in comics the writing and art are too closely linked to actually separate:
In comics, the art is the writing, and vice versa. Panel layout, pacing and visual rhythm, color, the expressiveness of line: these are all inseparable from comics storytelling, in the same way that you can’t separate Nabokov’s prose style and character descriptions from his writing, or the mise en scène and editing from the “writing” of a movie.
So, I'd take this, add it to my point about film critics and actors, and tie it back into what I was saying about film performance back in this post on Payback. We like to be able to separate out a movie performance from the script and from the directing, but any kind of separation like that is really artificial. Now, there's nothing wrong with isolating one of these elements for the purposes of discussion, analysis, etc., but we shouldn't forget that when we're actually experiencing a movie these three things - along with lots of other things - design, cinematography, staging, etc. - are completely intertwined.