RyanRowland:
I would assume that whether or not a reader comments on a story would depend mostly on how much he/she enjoys it, regardless of the length. But a longer story with more character development, etc. usually gives the reader more selection of what to comment on and makes it easier to find something to say.
That's certainly true, I'd figure, that both how much a story is enjoyed by the reader and its length would be factors affecting how long and detailed his/her comments on it would tend to be.
Of course, I make a significant effort in my commenting on stories, hence my comments on ones that I especially appreciate tend to be quite lengthy, even including the quoting of particular parts of them that truly reverberated with me and reacting to those quotes from the account. (This was notably true for the recently-completed F/M spanking story Challenge, since I'm an F/M afficianado and thus there were several entries which I pretty much became enamored of.)
Repeating other commenters' impressions doesn't bother me though, I'll sometimes even refer to an earlier comment that I'm agreeing with. Obviously, that's not an issue with the Challenges, since everyone else's comments are 'invisible' to me until the voting is completed.
From an author's perspective, I'd venture that any comment is better than none at all, although personally I'd assert that the in-depth ones are the most highly preferred...

--C.K.