My personal preference, which I believe it is shared by many who post in the galleries, is to send suggestions in a private message rather than publically posting UNLESS the person has specifically asked for constructive criticism. If it's a very, very minor point to be made, and it's included in an otherwise very positive comment, publically probably is fine, but it sounds like you are interested in ripping people a fair amount and all you'll accomplish is creating alot of hurt feelings. Most people don't like to be criticized publically but are more than happy to listen privately, particularly if coming from someone they respect. I'm not sure what your motivations are for wanting to do this, but those are my two cents (worth just about that much). Merry Christmas & Happy Holidays to all here at Renderosity!

Check out my full gallery at Cliff-Dweller Artworks