I guess that part of the problem is that one can't choose one's family...with your own parents, you get used to being around them, even if you are on a different wavelength, and it's easier to ignore/answer back parents if you have had practice in your teenage years.
If the PIL are annoying, it's not the same - first of all, you resent spending time with them, because they are not even your responsibility, and secondly, you feel as though you have to bite your tongue for your DH sake.
From their point of view, the MIL has exactly the same biological relationship with the child as your mother, yet doesn't have the relationship with you to get as 'close' to the pregnancy and the baby.
Also, many people are 'problem solvers', and will automatically offer 'advice' if they see your baby not sleeping/eating etc, and there is probably always something going on with a baby. They forget that their advice might be 35 years out of date, and see it as 'common sense' in the same way that we take a lot of stuff today for granted as common sense.
I don't have a baby yet (I am pregnant), and live far away from both sets of parents, but I am sure they will do my nut with well meaning comments , but I hope to just say "well, you know what it's like - they change the guidance every five minutes, we'll probably come full circle one day!" - this might appeal to MIL who is a teacher, and knows well that stuff goes round in circles, and 'new' techniques in the classroom have been seen before a few times in her career.
I don't expect much aggro, as neither set of parents judge our choices too much, but we all have an opinion on child-rearing...I was quite surprised that my step-mum was constantly commenting 'oooh she's having a tantrum!" and looking shocked when we were visiting friends with a 2.5 year old . I had to laugh - that's what a lot of 2.5 year olds do, and as far as I am concerned it's a normal part of development...not an issue of parenting. The parents dealt with tantrums really well...she just seemed shocked that the child should be having a tantrum...as though it was a sign of naughtiness, rather than a normal expression of frustration/annoyance that the child hasn't yet developed the skills to understand or manage!
When we have ours,I want them to understand that as this is our first child, we are finding our own way of being parents, taking on board all the current advice, adding a dollop of common sense and 'anything for an easy life', and finding what works best for us. I hope they all have strong relationships, and we can send kids off to visit them when a bit older, and when they are older, I don't want to be dictating how GPs care for them whilst looking after them on holiday anyway.
Massive post - you can tell I have read a lot of MN for someone who doesn't have kids...I suppose TTC for many years gives you a lot of time to crystallize what you think you will do when you have nippers