Authors, reviews, and professional courtesy

I love to read as much as I love to write, and when I finish a book I tend to want to review it. Not an issue in the past, but now I'm wondering...if one becomes a published author, is there a code of professional courtesy that says you shouldn't put down another author's work? Like physicians not badmouthing each other in front of patients because it reflects back badly?

Obviously there's no downside to praising something good and encouraging others to go out and read it. And some authors are just so gifted that they are reviewing from the artistic high ground. One would have to be grateful they even bothered to look at one's book. But what about those of us with modest writing skills?

Now a highly established writer won't care what I say. They get hundreds of reader reviews. Sir William Golding, were he still alive, would care less that I hated Lord of the Flies. (Amazing writer, which makes his pessimistic account of human nature painful to read for an unrepentant optimist.)

But what about someone less established, or in a field similar to my own books, where readers might care about my opinion, and maybe give it a little more weight now I've published? Do I need to only write the positive reviews and let the negatives slide? Or is it possible to write and critique without conflict? Is it possible to comment negatively on work by a more gifted author and not have it look like hubris? Personally, as a writer I'm still at the stage where every review from anyone helps. I've gone through all my unpublished manuscripts twice since my two books came out, emphasizing positives and hunting down flaws that were mentioned in reader analyses of my writing style. But a bad review from a name I recognize would sting more. So I've wondered...

For now, I figure I like most of what I read enough to give three stars or better, so I have plenty of positive reviews to keep me busy while I ponder the negatives.