Because I'd go with unique for better or worse. This is something I had a discussion about with my friend Troy, who will soon start writing for this blog (look out for our double review of Guardians of the Galaxy soon enough), recently regarding another song and he dropped this gem of a description that fits this perfectly as well: it has vocal character. Sadly I think a lot of genres are lacking exactly that. There are plenty of people who are good at what they do, there are a lot of James Labrie sounding prog-power metal vocalists, there are a lot of guttural growlers and every season of American Idol has at least a hundred girls singing an incredibly self indulgent RnB version of Amazing Grace.
Then, however, you've got the odd ones and that's where, for me, things get really interesting. Bands that have that aforementioned vocal character, even if it isn't pitch perfect every time or if it's an acquired taste (I enjoy acquiring my taste and I will continue to do so). Ranging back to the 60's with vocalists like Bob Dylan and Janis Joplin (further back too with Link Wray and more but decided to go with more pop culture relevant names) and to our time people have been interested in voices that are unlike the standard fare of the era but somehow I feel like some genres aren't embracing it like they should which brings me to the overall point:
Vocal character in technical and/or heavy music.
A) Being instrumental (not that it's always a compromise)
B) An OK vocalist
may be pretty tempting but if more bands were willing to take a risk it really can pay off big time. Like Daughters did: started out with fantastic grindcore on Canada Songs, followed it up with this:
However, just copying Patton doesn't really constitute as a great experiment now does it?
To break away from the really odd stuff for a while (before promptly returning to it later on) and visiting the heavier end of the spectrum even brutal death metal has recently seen a really successful experiment with different vocal approaches. What experiment you may ask? Cattle Decapitation's latest album:
Tell me because I need to know!
PS. You didn't think I'd forget Sikth did you?