Good thread!
I am big on reading the person's post carefully to find out what she is looking for. If someone doesn't specify whether she is looking for blunt comments, affirmation, or whatever, I assume that I should just speak my mind (though courteously). If it were me posting, I would want honesty. I think that honesty is necessary to help us move forward with our style.
I always try to give reasons for what I say when I critique. I think this might help the situation that CocoLion was talking about, when someone tries new trends where figure flattery is given up to an extent. If I don't like the fact that the outfit is not figure flattering (and this would probably be my reaction, because that's just how I am), I would say something like, "Would more waist definition look better?" or "The top looks too boxy to me." That way the person knows that I am honing in on the figure-flattery issue and can dismiss my comment if she has already decided that she is OK with a less traditional silhouette.
I don't know people well enough on the forum yet to say whether an outfit is "them" or not, so I just view every outfit from my own perspective and experience, which is all anyone can do. I remember the thread Shannon posted, which you mentioned, Susie, and I was concerned about how that thread was going because if I recall right Shannon clearly stated that she was trying something new. But the responses were very heavily in the camp that the new look wasn't "her." And that is where I think we need to be careful--if the person states that she is trying a new look, then I think we need to look at the outfit with new eyes as well and critique the outfit on its own merits.