Interesting.
Basically I can sing along to three groups of music nowadays - just about anything played on the radio (i.e. pop&rock) between '96 and '98, any metal i've heard since 98 and anything I hear in the car nowadays with my gf.
I used to just listen to music for the hell of it, not really taking too much away from it as far as lyrics or actual playing went. I started to hear some Metallica when Reload released, and thus made my first foray into metal. I went a bit into some Pantera at that point, as well as some Dream Theater (which I highly underappreciated), scoffed at Megadeth in my ignorant Metallica fan-boy state, and claimed I'd listened to Maiden, when I couldn't name a single song.
In late '99, though, a good friend of mine heard some Dimmu Borgir, and recommended I download some. I did, found it a little harsh comparable to what I'd been listening to, and so I just basically downloaded about 2 songs from each band on Nuclear Blast and Century Media that I could find on Napster. At that point, I discovered Dark Tranquillity, The Kovenant, and Hammerfall. The last of those was mostly just humorous for me, though I still found it entertaining musically. The other two really defined the way in which my music tastes grew from that point on. From DT, I went to IF, then later to Opeth, though more to Katatonia. From the Kovenant, I really hit Dimmu hard, got a bit into Emperor, but more into Borknagar. Between the two, somehow I heard of Therion, and that got me into a whole new realm, that I still enjoy today.
~Kov.