I'm not too hot on classifying bands into distinct genres, because there's so much variation within different musical genres. Slipknot and all the 'nu metal' bands out there are an example of this. They have a few major differences that separate them from most metal subgenres (ex: rapping/sampling, occasional pop-esque vocals). But taking into account their influences and other factors of their sound, Slipknot is pretty much fits into the metal genre.
Take some musical traits that define general metal bands: Use of power chords, palm-muting, syncopated riffs, emphasis on loudness, guitar solos, speed/"heavy" drumming, dark/gritty atmosphere. Slipknot incorporates all of those into their sound. The few differences that separate Slipknot from general 'metal' music don't erase all of those similarities.
They're definitely have more in common with metal than grunge or hard rock. It's honestly really hard comparing Slipknot to Iron Butterfly and Led Zeppelin or Alice in Chains and Nirvana. Since most nu metal bands were influenced by Korn, who were partially influenced by grunge music, I suppose grunge has been influential in nu metal music (not to mention most nu metal artists were probably teens during the grunge era). However, heavy metal is the largest contributor to the nu metal sound.
I mean, from what I gather, most people say Slipknot or other nu metal groups aren't metal because they hate them. If that's the case, then there's a fundamental problem with the reason for the classification. They're not "not metal" because they play "shitty" music. Because "good" is subjective. And they're not "not metal" just because they cater to angsty teens. Because fucking early Metallica wouldn't be metal, either. And I can't just call every artist/band I hate nu metal. You know, because then New Kids On The Block would be metal and that's just not right.