Hasn't DT been metal since.. Always? I know The Glass Prison was a song that had a lot of prominent fast and hard riffing, but I'd argue that it was still nothing entirely new for them, just the first time they'd put it all into one song. They've had the hard fast riffing since the first album, A Fortune in Lies anyone?
What about The Mirror? That song is more on the slower atmospheric heavy side but is still as heavy as they come if you ask me, that thing is just a string of badass riffery one after the other. (Similarly to how I see the structure of The Glass Prison).
I guess I don't know if this is controversial then, I don't exactly identify as a Metalhead or whatever but I got into the band through the newer stuff (BC&SL and backwards from there). By the time I got to Train of Thought / Six Degrees, I had a fairly solid impression of what I liked about DT. When I got back to FII things started to get inconsistent, and Awake / Images & Words were tough to get into, BUT what kept me coming back with songs like, NM, BMS, JLMB, LITS, TM, UAGM, TTT, PMU, L2L, CAIW, 6:00, Voices, pretty much, as long as it had some heavy riffing in it I didn't get too alienated. It took a bit longer for songs like Surrounded, The Silent Man, Wait for Sleep, Innocence Faded etc. to grow on me (and I doubt I'll ever like Space Dye Vest, possibly the one DT song I can't trick myself into liking). I've played keyboard (piano) all my life (or since the age of 7) and have even been fortunate enough to be able to start making a living off it in the last few years, but for some reason I lean towards guitar oriented songs with my favourite bands.
So what I'm saying minus the typical ramblings; I'm drawn to DT because of the metal elements, it's always been my favourite part of the band. Give me The Dark Eternal Night the second coming or Honor Thy Grandfather one day and I'll be a happy fan.