Something which struck me with particular force while reading Jenn's recent (very good) post about answering people who ask in relation to children, "Do you want more?!?" is that one thing we are typically not very good at is imagining the future as much other than a straight line extrapolation from the present.
This seems particularly important as regards family. Right now my ideas of what it means to be a parent are fully formed around what it means to be a parent of kids ranging from 8 through newborn. Living in close proximity to my father-in-law and MrsDarwin's older siblings on weekends since we moved to Ohio, however, it has been driven home to me how very different things will be in 10 and 20 years. Decisions we make now can have quite an effect on out lives ten or twenty or more years hence, but at the same time we actually have very little ability to understand what it will be like to be us that far in the future.
I tend to think of parenting in terms of having young children around the house, but looked at from the perspective of our entire marriage we will spend more time with adult children than with young children.