I've found this site genuinely very useful in terms of parenting advice especially in the baby years and I've also been thoroughly entertained by some of the posters here, many of whom are clever and funny and wise.
That said, one thing that I've learned is that many women are much more misandrist than I ever realised. This gets justified on here by talk about how men are hopeless or violent or whatever and of course some men are. But I find the way men are talked about on here to be often genuinely hateful and unpleasant and it makes me think less of women for it. I feel the same way when I read the awful stuff said in the manosphere about women. It's depressing that there seems to be an inability to recognise that men are just people. There's a tendency here to treat them as one uniform blob. You see it all the time: a poster complains about a husband who commits some minor offence and there will inevitably be the "I had one of these" response. One of these, like you're talking about an appliance not a person. And on and on it goes.
I also find it bizarre that on a site which seemingly does recognise the fact that biological sex exists and has an effect on behaviour, there's so little recognition of the fact that men, by and large, will not behave the way women do in every situation. Men's continued desire for sex is frequently treated as abhorrent, as disgusting or perverted. It surprises me that so many women get married, have children and then seem to think that's it as far as sex goes.
I'm also saddened by the number of women who don't seem to care at all about their husband's family and in fact often go out of their way to make that relationship difficult.
I suppose the TLDR is that reading Mumsnet over many years has made me realise that women are just as petty, selfish and cruel as men which is no surprise really but nevertheless makes for depressing reading sometimes.