the Why Wasn't I Consulted problem.
WWIC is the thing people talk about when they talk about nicer-sounding things like “the wisdom of crowds” or “cognitive surplus.” It has become the first thing I think about when I think about the web. I've spent a lot of time with users, and as part of various web communities. I've answered thousands of emails about things I built or said. Now, when I sit down to graffle, I start by asking: “How do we deal with the WWIC problem?” Everything else comes after.
Everyone who builds things for the web needs to keep this in mind.