Shh, shh. Let's keep the peace, gentlemen.
As a rule - not official, mind you (just a figure of speech) -, make the criticism constructive. Not all people are qualified as critics (even critics may not be qualified to be critics) but at least you can impart a level of help, if not friendliness.
Johns Volition, though you may not particularly like teh fonx's work, you might go a bit more indepth to your qualms about it. And checking for grammatical errors isn't valid in judging a piece of artwork. Judge it on its merits, which is in art.
If you just say, "What are you smoking?" or merely, "I don't like it," then of course the author/artist will be offended.