Pretty much what everyone else said. This very thread has some good examples. Looking at it, every post in here except these two would probably be considered constructive, given the context:
posts related to the topic with information.
Anything that is not related to the topic.
Those posts are not constructive because they are short and quite clearly didn't read the thread to see if someone had already said what they were going to say. In fact, I'm going to look over their posting history to see if they need to be banned.
