Her anti-confrontational attitude. It was a breath of fresh air to be with someone who had a pretty laid back attitude about things and didn't seek to make a fight out of the smallest things.
11 years later, still can't get her to properly communicate her own desires and/or gripes during an argument, or confront a family member about something out of line they've done.
****** Edit: Would like to add we are definitely working on this together. It used to be much, much worse. Since this seems to have resonated with a surprising amount of people, wanted to just add one little thing that has had an impact. ******
We now try to proactively have touch-base conversations. I've accepted that in the heat of an argument she will absolutely clam up, and that in the middle of a potential concern or issue, she may not immediately communicate her dissatisfaction. So instead we try to touch base every couple of weeks or so in an open and friendly conversation. * How you feeling? General happiness level? Anything bother you as of late? Anything I could be doing for you, or more of?*
That kind of stuff. Hope it helps.