What the fuck is wrong with Slipknot? Their lyrics are pretty bad but so are the lyrics of tons of metal bands.
From a technical standpoint, their riffs take just as much "talent" to play as most metal riffs. Granted bands like Symphony X are in a completely different world in this regard, but bands like Symphony X are the exception not the rule. Most metal riffs are easy to play.
In terms of the quality of their music, there is no objective measure of this (of course there really is no objective measure for quality of lyrics as well as difficulty of riffs). While I suspect there maybe people that genuinely hate their music, most of the Slipknot bashing I've witnessed results from an arrogant mentality held by most metal heads that anything nonmetal is inferior. Notice how the Slipknot arguments normally boil down to "Slipknot sucks because they're not metal." I respect anyone's right to claim this believe, as I understand that one's favored genre of music often evolves into a "way of life" and shapes one's outlook, but I feel it to be extremely limiting. I'm sure people that have branched out and embraced other styles of music will agree.