Fabulous topic!
I suppose I'll go first lest someone posts before I finish my typo'd response. I'm relatively new to this hobby, but the first real story I got to lose myself in is still my favourite. The person who wrote with me allowed and encouraged all the odd to burst forth and to be scribbled onto the page. Some of my best descriptive narratives came from that story, as well as some of the more thoughtful dialogue exchanges. Turmoil, at every turn, where the characters found solace within themselves, and the little places and side characters that had been thought up to make the other mind writing smile.
I've not had another partner, or story like that one. Don't expect I shall again, either....