A less-than-positive review?

Yes, I understand that an author has the final say over his own characters.

I guess my question is: is it rude or ungrateful to praise the plot of a story in a review while saying that the characters seem more immature than grown men that age usually are? (This would be at a website that encourages readers to write reviews.)

The story I'm thinking of is set in modern times in the USA, so I guess I'm expecting the characters to "act their ages"... Really, as I think of this I realize that it is all personal preference. This author's characters act less maturely than I'm expecting - but the characters are the author's, not mine, of course.

I find the characters' immaturity a little annoying - it distracts me from the plot just a bit - but the main dilemma the characters face is an interesting one. I expect to continue reading it mostly to learn how the author will have the characters solve their dilemma (not because I feel a connection to the characters).

Authors need feedback to improve at their craft. It's the gift you can give them in exchange for the gift they're offering to you.

That being said, not all authors really appreciate all feedback. What you might consider doing is sharing your thoughts on this story while being as diplomatic as possible. When offering criticism, it's always good to offer two positive observations for every negative your share. And try to express the negatives in clearly subjective terms so the author understands you're not making judgments about his skill, merely observing how something made you feel (e.g., "I thought the dilemma the author created for the characters was really clever and I'm fascinated to see how it will turn out, but the characters would have felt more realistic to me if their decisions appeared to better reflect the knowledge that I'm sure they have. I really like the characters but found it hard to relate to them when they did...").

You also might consider submitting your review directly to the author via e-mail first and asking if he'd like you to publish it to the public review section or keep it between the two of you, perhaps as part of an ongoing conversation about his story. That way he can receive the feedback that might help him develop his skills but can make his own decision about whether to have a partially negative review published to a public forum.

If you're going to give an author feedback, I think it should be framed in terms of whether something either worked or didn't work for you personally, and giving reasons why, rather than offering blanket pronouncements of "should" or "shouldn't." People don't like being lectured; doing so understandably gets a person's back up and then any exchange can easily become personal.

I'd also say that I'd be interested in hearing what the author thought about my observations; that would convey that you're going into this with an attitude of mutual respect and are interested in a productive exchange of ideas.

As to whether I give feedback at all, I only do it if I've found the work itself to be particularly to be to my liking, and I'll say so, either in general terms or in specifics. But if it's clear that it's just not my cup of tea subject- or execution-wise, I don't see any point in commenting.

Expressed this way, I'd expect an author who, regardless of age, was emotionally mature, to respond with at least a thanks for my interest rather than by lashing out or becoming argumentative. If the latter, though, I'd just move on.

I'll add that when it comes to offering specific advice or making suggestions, I'd wait to see if the author responds positively to my first message, and then ask if those would be welcome before actually putting them forth.