I agree that when you reach a certain level you do need to step back and look at what you are "Practicing".

Great Essay by the way.

Here's my 2 cents.

I have always beleived that the phrase "practice makes perfect" should be stated "practice makes persistance"

Meaning, If you practice the same thing/things perfectly then you will always(persistantly) play it perfectly. If you practice something"wrong" (I beleive there is no right or wrong in guitar if it sounds good--it sounds good) you will always (persistanly) play it "wrong".

Sometimes a practice session --as mentioned earlier- should be about what you want to get out of you time and what is fun.

Some tips passsed along to me and I have used in the past to help when you get stuck at a plateau (after 18 years you still find em') is to Practice with someone else who has aa totally different "style" than you, Who is better than you, who is not better than you, Practice in a jam setting (this will help you react to other peoples styles and help you develop a sense of "groove").

One thing I noticed that was not mentioned. No matter what or where you are practicing always make a recording.

Sometimes what your ear is hearing (while you mind is concentrating on that particular lick) is not what you are actually producing or vice versa and by going back to listen to your practice you may find out you like the way you sound or what sounded good actually did not sound good at all.

Another thing to help push you over the top and leave that plateau is to take a favorite lick or licks. Play it different speeds. Slow, Slow fast slow, medium, Medium fast etc.
Find at least 3 different ways to play that lick and play the different ways up all 12 frets (even learn to play it on open strings).

Try practicing your clasical pieces in a funk style, jazz style, blues/rock style, country western.

Play the same lick over and over again so many times that you force your mind to become so bored with it that you will subconciously play it differently which can sometimes be all you need to bring back the joy and fun

I'll get off my saop box now.