Ooohh this is a tough one for me! As a teacher [not of scrapping!] it is still in my basic nature to guide and offer advice. I know how to employ tact, but never-the-less, just the very act of saying something like... 'did you consider placing this photo here?' or 'this photo is great, could you draw more attention to it?' or whatever, will alert the poster to the fact you are being critical. Many of us are so proud of our own efforts and may not want to hear critical.
Part of the skill of scrapping is to develop our own eye for what looks good. The criticism to me, is not so much disturbing because it applies to my layout, but rather, I take it as a indictment that my own design eye is lacking something. Since I very much want to know my abilities to discern what is aesthetic pleasing is important to me, I would not enjoy knowing I perhaps got it wrong. On the flipside - sometimes beauty is in the eye of the beholder and who is to say I did get it wrong just because someone else says so?
I have to resist the temptation to critique when not asked to - and I am able to hold back. Man, my first layouts are just terrible, awful, horrid when compared to my recent ones but it was up to ME to figure this out, and figure out WHY. That is part of becoming a good scrapper - developing your own eye for beauty and finding out what you feel is important. I appreciate the fact I received nice, specific praise during this time. It encouraged me and pleased me. I was able to build on that praise and improve.
I have bought many scrapping mags and books and I know that these have helped me improve soooooooo much as have reading tutorials people have written. Perhaps a general post in the forums might be helpful like: "to all of those of us who find doing xyz so difficult, check out this tutorial!" or 'there was an article on balance in this book/ezine and it helped me fix up a layout I was working on! Check it out it might work for you too!" For me, this would scratch my critique itch without making any particular poster feel bad.
Finally, re comments in general:
So what don't I like? I don't like vague comments: 'this is great' [what is specifically?] however, I still appreciate people taking the time to comment. I also don't like when people comment without reading my descriptions of what I was trying to achieve or show with my layout. I once got a comment like 'I love xyz' where xyz was the only part of the layout I personally did not do [like a colour scheme challenge].
What do I like? Specific comments have helped me improve dramatically. They are to me, a way of critiquing a layout and saying something positive simultaneously. For example, "I love the way you highlighted the main photo" will alert the poster to the fact that a focal point on their page was a great asset to the layout. Or "great choice using xyz kit with these photos" will alert the poster to the fact that you like what they did with the layout, and not just you like xyz's work.
Sorry for the ramble! it is 5am here in Melbourne and I'm still on Vancouver time!

LOL
Great thread!