I don't think it's cliquey really. I've been mumsnetting for the last 18 months or so, and last year when I had a major MAJOR crisis, I found a number of the "regulars" really sympathetic, several going out of their way to help.
Then at the end of last year I said something a bit contraversial (to be honest, something I probably wouldn't have said to people's faces, but I believed it, still do). I was soundly rounded upon for my views, and to be honest didn't come on the site for a few weeks, cos I was quite hurt.
But I couldn't stay away for long, I missed everyone too much, and when I did come back lots of people asked if I was OK, and said they had missed me.
As for no replies on threads, well it happens. I have joked in the past about my ability to kill a thread completely, and sometimes it does feel like you're deliberately being 'blanked', but I think it's just that sometimes we all get a bit over-sensitive. I have certainly NEVER thought "Oh I'm not replying to that name", but do feel I should only respond if I've got something useful to contribute.
Oh, and for the record, I sometimes look at the bar threads, but seldom get involved. The bar regulars do tend to have their in-jokes, not meant to exclude anyone else, just that they've developed over a few hours/days.
Sorry if anyone feels excluded, I'm sure 99.99999% of us don't intend it.