I joined MN in 2003 when I was the only person I knew with a DC with ADHD and we were struggling with his behaviour. It was at the time very left leaning, very lentil weaving, very MC.
In recent years it has gone almost completely the other way. SPAG abandoned as "it doesn't matter" (when you only have the written word to convey tone/ meaning it really does) and people torn to bits for pointing out that it would be much easier to read a long post with paragraphs and punctuation.
It used to be quite niche where now it is mainstream. A military guy at work was describing something as "a work version of Mumsnet" the other day. The biggest change though is the constant filching by the media which really puts me off asking for help on anything really personal in case it's dragged across the papers or Radio 2.
As far as the polarisation, we get this in RL. Where I work people go off on holiday to Cuba or Borneo and think nothing of having 4 or 5 foreign holidays a year. Someone on my team has been just this year on a train across the Rockies for 3 weeks, 2 weeks in Orlando, a week in Las Vegas and several weekends away in France and the UK. That is normal in my office where most people are at a high level. We had a midweek stay in Newcastle as our main holiday 2 years ago (Mon to Fri) and people laughed when they found out 
. Basically we couldn't afford to stay in Cornwall or Devon.
In contrast, DH works in Retail. His colleagues go on holiday in a caravan in the UK or camp in Northern France. They didn't think our going to Newcastle was odd at all.
It is perfectly possible for normal people to be at either end of the spectrum. The only thing I do find odd about MNetters is how many people live with a partner and children. I can only think of a couple of people who aren't married at work, and they tend to be very young and pre children.