I believe it was Dylan who said something to the effect of: "I had to think up everything I ever said to you." I believe that thoughts between friends flow freely; that they don't have to spend a lot of effort on trying to figure out what to say or how to say it. The level of trust and honesty between them allows that freedom.

I am not sure I could write a shopping list of other features of a good friendship. Communication of the sort I have described seems to me to be the foundation for all the other things I might list.