Nananina, I understand you want to say something to balance the criticisms of mils, or put forward a different viewpoint. And you mention the age-old troubled relationships between dils and mils, which indicates that these situations have not only gone on a long time, but also that there's no real sign of a quick and easy solution. But unless I've missed them, you don't seem to respond to people who have similar kinds of problems with their mothers, or siblings, or whoever.
When you go on to say that mils can't do right for doing wrong ? if someone posts about a GP not being interested in the GC, that's one kind of person; if someone posts about a GP coming round too often, that's a different kind of person. And those different kinds of people can be parents as well as parents-in-law. There often is a problem of communication ? it can be much easier between people who have known each other for a long time, than between people whose connection has been relatively recent. But there is often an element of hierarchy. A person (and let's confine this to a mil) who has raised children, run a home, etc., could have lost sight of when she was young and a new mother, but instead see situations through much more experienced eyes and thoughtlessly intrude. But a lot of the mil posts here are from dils who either want to let off steam (and try and prevent a family row), and/or seek advice on how to deal with a difficult and emotional situation. Many responses are bound to be from people who have or are experiencing similar situations. Mumsnet is essentially a self-help group, and that works best when people facing similar difficulties get together and share their own personal stories, and out of that can come some good coping strategies. Just because others join in, it doesn't mean that it's a "mil-bashing" thread.
From the little I've read of the "Stately Homes" thread, it's pretty obvious that there are some very toxic parents out there. Some of those parents, by virtue of statistics if nothing else, will be mils. For someone, especially a young mum, who has never experienced that kind of toxic environment, it can be very difficult to know what to do. When you say that children have the right to enjoy good relationships with all their extended family, I'd agree with that point to a large extent, and I expect that everyone else would do also. But sometimes that right can be impossible to achieve, if some of their extended family cannot or will not understand what their role in that child's life should be. Overbearing and interfering relatives do exist, and sometimes it just has to be accepted that some of these will not make the effort to understand what their role should be, or modify their behaviour.
Regarding your last point, which I read as a warning to mothers of sons, I don't know about you but I personally feel that the mothers of sons who post here about their difficulties with their mils are already very likely to see things differently in the future, because unless they go on to have selective amnesia, they are more than likely to be sensitive and supportive mils themselves. Chipmonkey makes that point exceptionally well. You mention that your mil was a bit of a pain ? did your experience help you learn to be a good mil yourself? Do you think you wouldn't have been such a good mil if you hadn't had to deal with you own mil problems?
Finally, dils post here because they need to ? if they didn't, they wouldn't.