XP is paradoxical; everyone agrees that it is a pretty meaningless number,and yet (I am convinced of this) it is the key to the success of PM as a web-based community.

For the thermodynamically inclined, I offer this analogy: XP is not a state variable like energy or entropy; it is more like heat or work, in that only the "local" changes in it are meaningful (in the way they affect behavior); the net global sum of these local changes, by itself, is meaningless.

Or a more familiar analogy: XP fluctuations are the equivalent of the many smiles and frowns we get from people around us throughout the day; these gestures have an impact (often unconscious) on how we behave minute-to-minute and day-to-day, and in this respect they matter a great deal, but it would be foolish (or so I think anyway) to attach importance to some cumulative net number of smiles minus frowns that one has garnered through life, even if they were at all quantifiable.