Diversity - I can honestly say I have changed some of my opinions by diving into some of the more "heated" discussions on Mumsnet!
Friends - moving house/area was extremely stressful for me, I can honestly say the experience would have been an awful lot poorer if it wasn't for the two lovely mumsnetters near me now. In fact, one of them (you know who you are!) practically held me together during my last pregnancy - still owe her so much for that.
Laughs & Tears.
Classic threads such as the Strawberry!
Shared experiences - it can be such a relief when you think you're all alone to have someone post "that's happened to me too".
I think it's worth noting that pretty much all the close-knit internet communitites go through these sorts of phases where everyone gets a bit ratty. You just have to ride it out - the community does pull through the other side and is usually stronger for it. Don't get too disheartened