I've always loved the implication that if a person isn't completely enthralled with every bit of a Sondheim show, he/she must not be deep enough/smart enough/whatever enough to appreciate the inherent brilliance of every last millisecond.
Parts of Shakespeare's plays are boring, and dare I say, parts of some of Shakespeare's plays are pretty bad. I'm pretty sure that Sondheim does occasionally have his not-so-great moments as well.
I hesitate to mention them as I will instantly be labeled a plebe, however.
'"Contrairiwise," continued Tweedledee, "if it was so, it might be; and if it were so, it would be; but as it isn't, it ain't. That's logic."'
~Lewis Carroll