Films often have commentary from the director, producer and actors. This commentary can sometimes be both interesting and enlightening. I think it could be very worthwhile if some level designers did the same.

I envisage it working something like this:
The designer records a walkthrough movie of the level and he or she would commentate on aspects of the level. The level doesn’t have to be actually played or if it was then using God and Fly mode may prove better.
The kind of commentary could include why they chose a particular texture or what inspired the basic layout etc.
If you don’t initially see the benefits try to look at it from a different point of view. There are many players and developers who would enjoy and benefit from see the “why” of level design not just the “how”.
I accept that not everyone will be interested in the commentary and that it will work for only a few levels and need the right sort of designer but I think as a concept it would prove popular.

Looks like somebody else had the same idea:
F.E.A.R. Directors Edition