I’m smart enough not to respond publicly to reviews, of course; that pretty much never ends well. But if you want to know which ones get up my nose the worst, it’s the ones that make unfounded declarations about what was in my head while writing. If you read a particular thing out of the story, fine — far be it from me to say ur doin it wrong. But please don’t claim you know why I did things that way.
Mind you, the line between the two isn’t entirely clear. Sometimes — as Kate’s contrasting examples show — a lot of it comes down to phrasing; if you say “it seems the author felt X,” that creates a different impression than “the author felt X.” This is one case where I think it’s a good idea to use qualifiers for your assertions, even though in other circumstances it’s better to just say things directly. And, of course, if you’ve been reading my blog or an interview with me or whatever, anything I say there is fair game for use later; your review can say “because Marie Brennan is concerned with not taking events out of the hands of the real, historical people who were involved, she does Z” — though even there, it would be better to say you presume there’s a causal relationship, because when you get down to it I may have forgotten my own agenda and done Z simply because it looked nifty, or the rest of my plot required it.
Talk all you like about the product. What you say may sound very odd to me; I may blink in surprise at the cool thing I apparently did without noticing, or wonder exactly what novel you read, but in the end “the book” is the product of a chemical reaction between the words on the page and the contents of the reader’s head, and I only control one half of the ingredients. The contents of my own head, on the other hand, do not belong to the reader, and so I would prefer that reviewers phrase any speculation as speculation. Don’t be the guy who went around telling people what Ursula LeGuin “intended” with the Earthsea books. Don’t presume to speak for the author. If I’m going to bite my tongue and not tell you how to read my work, don’t tell me how I wrote it.