Evanescence: The lyrics, Amy Lee's vocals, and in the time of their album "Fallen", the guitar work, especially in the song "Tourniquet". My first Evanescence song was "Missing" but the one that got me into Evanescence was "Bring Me To Life".
Within Temptation: I originally liked them because they were kind of like Evanescence, but I've grown accustomed to their style and Sharon Den Adel's vocals are higher pitched, something I really like, and their music is more theatrical. Also, I heard my first Within Temptation song ("Memories") without knowing the band name and loved it. I was always capable of getting "Jillian" stuck in my head back when I was first getting into it...
That's about it.
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"...The heart's desire is found... in an unexpected place..."
Tailchaser's Song" by Tad Williams