SNM:
Speaking as an author, I would much rather have people point out errors and other complaints in the comment section, so that others can weigh in as well and it opens a discussion. I like this.
I'm in agreement on this concept, although by "errors" I don't mean minor 'typos' or misspellings, but rather problems like sudden, unexplained changes in verb tense or point of view, which can significantly affect the reader's comprehension and hence lower the literary quality of the work.
As for "other complaints," those could be criticisms about plot elements or characterizations, but they also might simply be reactions to particular events, attitudes and behaviors, which wouldn't strictly be complaining as opposed to merely commenting.
While I'd appreciate being able to occasionally engage in a 'group discussion' type of interaction in dealing with a particular intriguing or controversial story, this Library doesn't appear to be structured for it--at least it's hardly encouraged, except for two-person (reader-author) private exchanges.
Personally, I don't at all mind commenters being critical toward aspects of my stories, as long as they're not point-blank insulting...

--C.K.