Just figured out what it is about Chad Kroeger's singing that's so dead-sexy. It's the way he treats vowels. I'm listening to "See You at the Show" (he's the lead singer of Nickelback, FYI), and it struck me that he draws out vowels in a very unique way, like he's unwilling to let them leave his mouth. He hangs onto them a little longer than most singers would, and it just comes out all caressing and seductive.
Speaking of Chad Kroeger, I wish I had a better picture of him to put here, because he's my ideal Sirius Black. Whenever I'm reading the Harry Potter books, I always envision Sirius as a dark-haired version of Chad, all skinny and lanky and slightly sinister. Not that I don't love Gary Oldman, but he's not who I see in my head when I imagine Sirius.