I agree with the OP. It is horrible on here sometimes. People sneering at peoples choice of children names, saying if you say 'tea' instead of dinner you are 'common,' and it's 'very working class' to say it, saying regional dialects are only used by 'chavs' and middle class people don't have strong regional accents, and only under-achievers rent their home, and so on and so on. (Whilst obviously mentioning that they have three university degrees, and a massively successful high-flying professional career (that seemingly allows them to spend 8 hours a day on mumsnet amazingly!) And of course they have cleaners, and a gardener, and a maid, and a butler, and a nanny.
And the amount of 'teachers' and 'doctors' who are always on here amazes me, and also lots of people who work in law, who obviously know everything. And they think £60K is an 'average income.' 
It's a joke to be honest. Most people on here are low-grade admin at best, with an average 3 bed semi, and an average income. OR they are stay at home mothers who WERE in that kind of job. Nothing wrong with that of course, but why try and pretend you are something more/better/more superior?
Some women claim to be on sabbatical from their hugely successful highly paid career, while they are looking after their kids. But NO-ONE who is a very successful professional on a 6 figure salary, gives up work to raise a family.
Every woman I know who is a hugely successful professional, or is highly successful self employed business woman, has 3 or 4 months off, then goes back to work. She has a nanny or a creche at the workplace. And she most certainly does NOT spend half her working hours on mumsnet.
And as several posters have said, these smug faux middle class 'highly qualified professionals' are ALWAYS a size 8, and think anything over a size 12 is 'fat,' and they ALWAYS cook from scratch! And they always have husbands who are on £200K, with a very long commute, and the husbands have such an exciting career, that the wife can't book any holidays away or anything, because she never knows where her husband is going to be from one week to the next.
And people only sneer at 'middle class' people, if the (faux) middle classes try to sneer at others. I also agree with the poster who said being financially well off does not automatically mean you are middle class/classy. I know some lovely, polite, classy people who have very few assets and very little money, and some horrible, arrogant, nasty people who have lots of material goods.
The ones who make me laugh are those who grew up in a working class home, and think because they went to uni, and are now on £35-40K and have a £250K mortgage, that they are now middle class!
But yeah the threads that start 'I hate it when people say.....' are just unpleasant, and snobby, and obnoxious. As are the posters who start them.