I think I've just solved ALL of mumsnet problems. How about there's a 24 hour delay on everything you post with a proper review button... that way things posted in the heat of the moment could be easily retracted in the cold light of day with everyone being none the wiser. Of course, all conversation would be completely destroyed, but it's a small price to pay
I have to say that it's the heated discussions that have really taught me lots. I didn't agree with lots of what was said on the downs syndrome article in the guardian recently, but boy do I know so much more about DS now. The GF threads, the MMR threads, all of them actually have a very high quality of debate in them which really helps stretch what I think about issues - yes, they can get a bit hot in there, but I can choose to bow out when I can't take the heat anymore.
Parenting topics are bound to get heated. It's something we're all highly passionate about - hell, even some people who don't have kids get passionate about it - just do a google search on anything related to "childfree"
Part of that passion is that it's very easy to feel defensive about our own particular stances. Sometimes it does us good to get defensive - it makes us think more carefully about just what it is that we're defending. Sometimes it causes devisiveness and harm (usually accompanied by the "lalalala I can't hear you!" approach). It's a very grey and faint line between the two.
As for the silly threads, they're a natural part of it too. We're all stressed enough in our daily lives that we desperately need to blow off steam from time to time. Some choose to do it here, others elsewhere, but we all do it. I don't see that mumsnet is somehow an "inappropriate" forum to do this in if we want it truly to be a community - in some ways it humanises the person behind the typing and can reduce keyboard warrior tendencies elsewhere (all in my opinion of course).
See, the great thing about the internet is, that it's a very freeing medium. I can click what I find interesting, and ignore the stuff I don't with far more ease than I ever can with the telly (which is why the telly usually stays off in our household and we have an always on internet connection). I actually read a lot of the special needs stuff even though I don't have a special needs child, simply because I'm interested. I don't bother with a lot of the social stuff, because I get my social stuff elsewhere, and I blow off steam on my weblog or in chatrooms not on mumsnet - that's my personal preference.