I don't think it's that hard to give constructive criticism. State what you like, what you don't like, and how to fix what you think is wrong. There's a difference between helpful comments, and rude, negative ones. With the Xena example above, try something like: "The bikini looks like plastic. A fabric or leather textured bikini would make more sense." I always plead for constructive criticisms for my work. Things like, "It sucks" doesn't help much, though.