I pretty much agree with Viking on this. I try to be as supportive of other authors as possible because I know what it is like to have people $#!+ all over something you've worked on. Even if someone's story isn't my cup of tea, I try to find something worth having a positive comment about or at very least I just don't say anything at all. If I have suggestions on how to make something better, I think Viking is correct that taking a kind and helpful approach is best. And doing it in private, probably not a bad idea either.
Likewise, I know I'm not a very good writer, in fact my English skills despite ostensibly being a native speaker are pretty crappy. So I welcome a certain amount of constructive criticism... as long as the 'constructive' part is there.