I think it can be a difficult relationship as a woman is often forced to cooperate with her mother law in a way that a man rarely is. Unfairly, IMO, it usually falls to the woman to deal with both hers and her DH's family, in terms of family get togethers, birthdays, Christmas, grandchildren, etc. So basically, the woman, a stranger, is coming into an established family and trying to negotiate situations that can cause a lot of angst and bad feeling. It's a very tough relationship to deal with as you don't just have to be friendly, you have to try to set a sort of working relationship between two families. If you get on fine with your MIL then it should work ok, but if you have clashing personalities, different priorities or just different expectations then it's a recipe for disaster.
Things were going downhill fast between me and MIL until I though "fuck this, why am I the one constantly organising visits and negotiating where people are staying etc, this is DH's family!" So I told him I wasn't dealing with it anymore and since then things have improved hugely between us. MIL was always a bit suspicious of what I said, assuming I was fobbing her off or that I really didn't want to visit, but now that everything goes through DH, she believes every word and doesn't get worked up. I can just relax and be friendly with her and there's much less tension between us.
As far as I can tell, in a lot of the situations that people post about on MN the MIL sees the DIL as the "gatekeeper" of her son's family and the DH has just completely checked out of the whole thing. What you have then is two people who don't know each other very well trying to gauge each other and establish boundaries. It isn't fair at all and I think if more men stepped up and took responsibility and smoothed the way between their wife and mother things would end up a lot better. I think women need to put their foot down and say "she's your mother, you deal with her."