Some time ago I came up with this on the IRC:
Imagine that you have a puzzle, and the picture in the box is a boat on the sea (yes, one of these solid ones).
With Pylons, you will begin to look at all these blue pieces and put them together, sometimes ruining, because you do not know if they are the sky or the sea.
With TurboGears, all of the blue ones are already done, and you just go and work on the ship. But if you are interested in making part of the blue, you go and cancel this part of the puzzle and build it again *.
* here the analogy breaks down a bit, because the replacement component does not need to be built from the same set of pieces :)
source share