Below is a great little Q&A dialogue from www.gettingthingsdone.com:

Q: What’s the one thing that we do that gets in the way of us being productive?

David Allen: It’s not one thing, but five, all wrapped together: People keep stuff in their head. They don’t decide what they need to do about stuff they know they need to do something about. They don’t organize action reminders and support materials in functional categories. They don’t maintain and review a complete and objective inventory of their commitments. Then they waste energy and burn out, allowing their busy-ness to be driven by what’s latest and loudest, hoping it’s the right thing to do but never feeling the relief that it is.