I'll come at this from a slightly different angle. I'm on another forum (not writing related) that has a very large group of posters (like, several thousand). This forum has been around for many years, and as such, some of its members have become very familiar with one another, our posting styles, and friendships have formed. On this particular board, there used to be a section for "swapping," which I suppose is sort of like critiquing, but with actual goods in exchange for money or other goods ;)
We eventually had to do away with that section because newer posters would come into this very generous, giving community and "swaplift," or integrate themselves just enough to get someone to send them something (or in a few rare cases, LOTS of things), and then they'd cut and run. After that, it was still a generous community, so if someone was having troubles, many times people would converse via PM and send them care packages. But then we'd get new members coming in and posting a sob story, trolling for goodies. It completely ruined the ingrained trust of the board and made older members instantly wary of new posters with certain styles -- posting too much personal information too quickly, jumping into established topics without properly brushing up on the culture of the forum first, not getting to know anyone before trying to buddy up to the "oldies" and expecting special treatment, using phrases or terminology that isn't approved of (phrases like "that's gay" or "that's retarded" are severely frowned upon there, so if someone drops either, there's a crapstorm).
Anyway... what I'm trying to get at is that boards that have been around a long time and have very established members DO tend to be very wary of "newbies" for a number of reasons. There's usually a lot of history, fair or not, behind it. A lot of people join writing boards for a quick critique that they never return, or they spout off an opinion that the board as a whole generally disagrees with and they get pounced on for it. It's something the "oldies" have seen time and time again, so they tend to think the worst and expect to be proven wrong. Or someone asks the same question that others find annoying and have answered a thousand times, so they get snarky, even though the person didn't really know better. Which isn't fair, but that's the mentality. Generally speaking, lurking and learning the culture and general stances of the board is a good way to go. Start small and safe, build up a post count, get to know the members. Baby steps.
All that said, it's still not really an excuse for people to be outright nasty. I think this community is wonderful about being considerate about the way they phrase things, even when they don't agree with what's posted. I hope that we can continue along that line, even as we become the "oldies" and newer members join the board.