Playing lead guitar is about composing music spontaneously, and the key to creating a memorable solo is to consider its structure within the context of the piece's melody, rhythm, beat, and general mood and tone. The main difference between shred guitar and other styles is that shred can potentially throw these considerations out the window, and allow the player to simply focus on spiralling up and down scales at breakneck speed; it is at this point that shred becomes meaningless wankery. Granted, there are highly accomplished "shredders" (eg. Joe Satriani, Steve Vai, Marty Friedman) who incorporate the aforementioned considerations into their highly technical scalar approach to playing, and they deserve all the accolades they get. What brings shred down in the eyes of many guitarists is the vast body of shredders who learned to play lead guitar by copying Kirk Hammett's widdly bits, and produce solos by replicating these sections in the appropriate key (Zakk Wylde being the chief culprit).
Shred is fine, as long as utmost restraint is exercised when playing it.