I don't think it is the 'norm' as such, OP, you're going to get a skewed version of events on MN because people rarely post about how well they get on with their in-laws as a topic of conversation. Perhaps someone should create an 'I love my ILs' thread to counterbalance? 
I have a son and I do worry sometimes that I won't ever be able to do anything right and will piss off my DIL (or SIL?) when he gets married. I don't even want my boy to feel uncomfortable around me or like he can't bring his partners home because of an 'atmosphere'.
I think I will take tips off of my ex's mother. I still talk to her now as my friend, even though me and her son are loooong finished. She was the perfect MIL material - made me feel welcomed, loved and part of the family from the get-go, included me in events (including holidays and meals out), supported me through tough times in my life, and generally let me and her son 'get on with it' and have our relationship without meddling. Sure sometimes she'd overstep a boundary but we were upfront and honest with each other, because I knew that telling her 'that's too much' wouldn't result in her bearing a grudge for days after or throwing a manipulative strop. All of this was alongside, and complimentary, to the relationship between my ex and I - infact in the end I only really stayed with him to keep my friendship with his mum intact (obviously not healthy so I ended it and then a few years after when all was healed, got back in touch with my friend/his mum, and we have a good natter on our own terms.)
She was/is such a cool, 'trendy' sort of person - active, fashionable, looked after herself, had her own hobbies/life - the sort of person you can have a laugh with, and we get on so well.
My current MIL is very different. I do not dislike her as she has lots of great qualities - she's generous, has raised a great man in my DH, and despite a few problems of her own tends to grit her teeth and get on with life in a determined fiery way which is admirable. She is quite clever as well. That said, I've never felt truly accepted or like I fitted in with the family. I always get the feeling she and I are just too different; our likes and dislikes are very vast, our priorities are also totally opposite.
I also get the feeling she is quite resentful of me and acts like she is jealous of the time I spend with her son; I think she thought he would still act how he did when he lived at home when we got together (staying up there with them over the entire xmas break, on his own without me, for example) when really we were trying to find our own way and needed some space to let our relationship grow. We had just moved in together and were trying to get used to the costs of living, and were working full time so didn't get much time as a couple. As we wanted to spend just one day as the two of us, MIL wrote me a mean-spirited letter saying how I was disturbing their 'family cmas plans'. It was the unwillingness to compromise from her that got my back up - and she still tries it on a bit now, despite both me and DH asking her to back off a bit.
I do not expect my DH to 'go up against' his mother in a fight or family feud - but I do expect him to stand up for my feelings and my importance in his life, as I do for him to my own family. If there is any negativity for any reason I do not expect it to be ignored.
Sorry for the essay - just trying to see OP's point of view and answer a few of her questions from the perspective of someone who's had it both ways :)