I hear little to no death metal in their sound. Nu metal at worst, proggy groove metal at best. It's like Machine Head trying to be interesting, and failing miserably. I hold it in the same regard as recent Sepultura; modern trash riding a trend. Everyone I've talked to about Decap basically calls it nu metal, but I've also heard groove metal and djent, all of which fit. What does not fit, is death metal.
U_C asked for a less elitist-y view on what's wrong with them, and what I said above obviously reeks of elitism. Musically, what I find wrong is this: the vocals are no longer death growls, they're weak hardcore shouts, again like what happened to Sepultura. The music is no longer rooted in complex, winding riffs like they used to have, instead opting for boring, chugged, "off-time" palm-muted Machine Head rip-offs. The drums are no longer insane blasts and lightning fast double bass, having been replaced with dulllllllllll as fuck Vinnie Paul worship. I legitimately hate what Decapitated has become, and that's coming from someone who thinks Winds of Creation is a masterpiece. I hope that explains why so many fans have changed their opinions on the band and why they are NOT death metal.