It's easy to criticize, but hard to create; critical responses are the low-hanging fruit of intellectual discourse.
This topic is surprisingly well timed for me; just the other day I was reflecting on the fact that most times I'm moved to comment on something, it's sharp criticism. I was wondering if I've just got a massive negative attitude; I would prefer to be more positive in my approach to life. But this is the reality - most of what I'm criticizing really is stupid crap. And when something comes along that's actually praiseworthy, I acknowledge that accordingly too. It's just that those events are few and far between.
As for ideas presented by newbies, my initial reaction depends a lot on presentation. If it's an overly excited, enthusiastic "hey guys, look at this amazing new thing I've created" I'm more inclined to judge it harshly. If it's a more measured, "I think this could be interesting" I'll be more inclined to study it and evaluate it supportively. The message defines the messenger, and sometimes the messenger deserves to be shot.
Someone above commented about negative critique reflecting arrogance and ignorance from the critiquer. I think the case I just described is the opposite - it is arrogant and ignorant of the newbie to believe they've discovered something that no one has thought of before. Most of the time you can point them to previous solutions with no more than a few seconds' thought. Most of the time their approach demonstrates that they don't understand the system they're working with, and don't even understand the problem they think they've solved. A cautious presentation indicates that they've thought it through and are aware of their own limitations - I always encourage thoughtfulness. An over-excited presentation generally means the opposite, and I will discourage them strongly and harshly. There's no excuse for ignorance, not in this age of ubiquitous access to information.
This isn't kindergarten. Not everybody deserves a gold star.
Another common trait of the 'troll' to which I refer is that they often have some lame excuse for not getting their contribution to the project in good order on time- yet they are perfectly willing to stand in harsh judgement of others. The most startling cases I observed were as a faculty member. Professors couldn't be bothered completing their department work on time- but accepted nothing late from students.
The pattern seems to be that these folks are not good at finishing. Big ideas? Sure. But the devil is in the details, and they tend to shoot down others in the details, while if they had to climb the same path they would have never made it themselves.
I don't know much about psychology, but if there is a false arrogance bred from insecurity, then that's what I think I am trying to relate.