Several conflicts recently on YWS have brought something to my attention -- exactly how responding to a piece a member has posted can be in the best way beneficial to the writer.
Now, in my experience, there are a few different ways people respond.
1. The Praiser
Look at for phrases such as "I loved it!" or "I can't think of anything wrong with it, it's brilliant!" and the ilk. This is probably one of the most common types of responses on writing forums. In some cases, people just don't know what to say and find the piece in questions brilliant, and find themselves too inferior to criticise. In other instances, people are just trying to be nice and encouraging. However, the real problem is, praise has never spurred anybody on to improve. Calling something "brilliant" psychologically is telling the writer there is nothing to look at that is bad, and that there are no sections in which the reader felt were flawed. Hence, they assume, there are no improvements to be made. While obviously it can improve a writer's confidence, it can hinder their potential improvement. Probably no writer would ever consciously think their piece was flawless but a few comments like that can often mislead a writer.
2. The Spoilsport
Similar to The Praiser, except the opinion is the opposite -- they think the piece is crap and not good at all. It has the same problem as the Praiser, because it doesn't help the writer improve -- there is no indication as to which parts were particularly thought of as bad (or there is a limited explanation). This basically means the writer both doesn't know what to improve and loses confidence in their piece. The second part is particularly bad -- reviews should always strive to be positive and uplifting, even when picking out bad things.
3. The Pedant
The Pedant is the member who quotes excessive amounts of different words, corrects spelling and random grammar mistakes, and doesn't do anything else. Basically they are proof-reading the document. While this can help the writer pick up things they may have missed, in the long run, it doesn't actually help the writer improve at all. There needs to be some indication of what they liked or disliked, and what they thought needed a look at. With a pedant there is none of this, only excessant small mistakes being noticed and quoted.
4. The Vague Commenter
Pretty self-explanatory -- the member decides to tell the writer what they disliked and liked, but never really elaborates. Things like "The characters were good but some were a bit cliche and boring" or "Some of your grammar was off" are the sort of comments that appear in this category. While they, on appearence at least, seem helpful, they are too vague and not specific enough to help the writer locate these supposed problems. The Vague Commenter needs to eradicate their vagueness and particularly specify what they didn't like, with examples.
5. The Critiquer
Lastly, and best of all, is the critiquer. I reserve the word critiquer for someone who actually pulls off a complete critique. What this means is the member has done everything they can to help the writer find exactly what they didn't like or like -- e.g. "I thought your dialogue grammar was off. See *here* and *here* where your commas were in the wrong place. Take a look at this article that can help you understand the rules" They give exact examples. They remain upbeat in their critique and remind the writer about what was great, and how this piece could become much better if they were to follow the advice given. Their comments are intelligent, sound and often pick up things nobody has noticed. They pick up on plot holes, unlikely events and problems with characters. They critique every part of the story -- description, dialogue, action, plot, characterisation, etc. Now, obviously, not every critique is like this -- but most have some of this and are the best way to improving a writer.
While some might say they are to harsh -- the thing is, nobody ever improved without being told what was wrong. If all we heard was "that's brilliant" all our lives, there would be no reason to improve, because, according to what we've heard, everything would already be right. And that's why praise, spoilsporting, pedantic comments, or vagueness aren't needed necessarily, while critiques are.
When I'm in the mood for a good, long crit, I do #5. But I'm guilty as the praiser too. (Sorry Claudette! 
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