See, I feel that early 2000's Heavy/Nu Metal can usually be distinguished simply by overproduction.
The studio has cleaned up so much, the instruments almost stand completely separate from one another. You can take each one out and still recognize the track, because they're all playing from their own room. There's just so much individual attention, it drags damn near needlessly at the details of an instrument that may have sounded better within that group, if you were simply allowed to play how you wanted to.
I also feel there was a big call to produce radio-ready tracks that would be pumped out at the speed of light. You can hear the guitar loudness is too sharp, or the drums are just a crisp & clear rendition of a slightly more sloppy version of the same track, or even just the vocals being too polished. There's no grit to any of the vocals, as they've all been washed over, and over again in studio. They sound almost isolated from the rest of the group.
Even black/death metal bands during this period are guilty. There's so much separation in sound, that groups like Immortal, Behemoth, Cradle of Filth and Dimmu Borgir, all suffer from this sense of overproduction.
Everything is too loud and clear, all separately from one another, that you can pick apart what you want to hear without ever even enjoying the entirety of the song. It never stopped me from enjoying these tracks, but damn it, I notice it way too much now.
Oh, and too many bands tried to incorporate a post-grunge type of vocal into their sound. Whether they sang, growled, or screeched, at one point or another, you would definitely hear this drawl to their vocals that would bring them ever closer to the realm of the grungiest. From Alice Cooper to ZZ Top, many groups who were producing at that time tried their hand at becoming more raspy, and draggy with their vocal sound.