Unfortunately, there are those for whom the definitions of "constructive" and "nice" simply allude them.  That and the fact that it is virtually imposible to ascertain someones tone when reading, we should be a bit more careful in what is said regarding someone elses work.  

When someone is asking a question in the forum that has been answered many times before, it may be "true" that the answer that comes to mind is "jeez, look it up" but to someone new or someone not familiar with the forums that idea may not have crossed their minds so the correct response would be to answer their question.

If someone is new at poser or for them poser is a slight hobby, their renders may not be great, they may struggle with lighting.  Responding to them as if they should know better or simply telling them a render is "bad" is not constructive but offering suggestions, nicely, is the way to go.

In the simplest terms, it's all about how something is said.  

...... Kendra