One of the things I stress in teaching how to critique, and to which I believe Critters owes much of its success, is delivering the bad news diplomatically.
Writing a critique is unlike most other forms of writing, and thus is often new even to the most experienced writers. After all, you're writing for an audience of size one, and almost certainly have something negative to say -- but rather than trying to persuade them about something, you're hoping they'll just hear what you have to say.
I've written other pages detailing the "why"s of proper critique phrasing (see It's not What You Say, But How You Say It). On this page I simply wanted to summarize the mechanics of what I've found make for more tactful and polite critiques. Before I get to the specifics, there are a small number of general thoughts to keep in mind when writing a critique:
Okay, details. Here are the specific phrasing issues I've noticed that separate the diplomatic critiques from those authors complain about as offensive:
There now, not such a hard list, is it? I know it's mostly a matter of treading lightly, but trust me, following the above advice will render your efforts much more successful!
(Oh, and the converse: When you're an author receiving a critique, don't assume the critiquer will have done any of the above. But do assume that everything they say is 100% their personal opinion. Look for what many people point out, and feel free to ignore what only one person says. :-)
I'd also mention that these sort of diplomacy rules are nothing I particularly invented: The esteemed Clarion SF workshops and those that follow its method promote civility as well. You will also find similar advice in the long-popular book on leadership, "How to Win Friends and Influence People."
So remember that a critique is not an editorial (meant to persuade readers to your view), or a critical review (meant to educate others whether they want to read the book, see the movie, eat at the restaurant, etc.), etc. It's a very personal, one-on-one description of your reactions, and thus has its own recommended style. Try it; I think you'll find it works.
(Next, there are some examples of problem resolution cases I've dealt with and specific wording suggestions.
When you're done with that, you can run your critique through Aburt's experimental Diplomacy checker to see if it finds any red flags.)