 Again, "I liked this" is fine.


More fondly known as "constructive criticism," negative reviews are just as important as positive ones, even if they do sometimes smart on the receiving end.  If you have just read a story which was good but you feel was marred by some aspect or another --- perhaps something was not canonical, perhaps the author spelled a character's name wrong --- you should not feel intimidated about pointing this out to the author.

To leave an effective critique, you must keep some things in mind.  First, you are not superior to the author, and the author is not superior to you.  You are peers and equals, perhaps not friends but certainly companions along the road of fandom.  Using a review to further a grudge, call names, or make yourself feel important is impolite and counterproductive.  As a rule, stick to language you would feel comfortable relating to your mother.  (Dr. Merlin is aware her relationship with her own mother is perhaps a poor example for this case, but rest assured Merlin's medications are being adjusted as we speak.)  Find something positive to say about the story at the beginning and end of your critique, even if you are merely complimenting the author's spelling.  Also, and this cannot be stressed enough, if you are nitpicking a detail, make sure you are right before you critique; few things are as embarrassing as pointing out that the author is misspelling a character's name, only to discover you've been doing so all along.  Fact check yourself, and you will come across as a reliable source of information.  Be truthful, be specific, and again, be polite.

Constructive criticism is often best handled via private email rather than public messageboard.  You can engage in a dialogue with the writer and perhaps the concerns you had will be explained as intentional on the part of the author rather than errors, at which point the discussion becomes a matter of what is and isn't effective storytelling.  As an example, years ago I received a critique on a story that used an obscure character from canon in order to show what was going on in the lead character's mind.  The reviewer suggested that no one would know who the character was, which made it less effective.  This was useful to know for character choice in future stories, but it was also not as relevant for the story in question as the intended audience was a small subset of the fandom who were very familiar with the character.  This discussion was vital for us to understand each other's position.

Critiques on small details (spelling, eye color, canon trivia) that can be easily changed, especially right after publication, are blessings on an author's house.  Critiques on large details (pacing, characterization, plot structure) are lessons for future stories, and are also blessings.  Critiques on small details for older stories, especially those more than a few years old, are perhaps not worth your time in pointing out. 