I'm holding off on my vote because I just don't know.
I like Dream Theater for the atmosphere their good music creates, and for their melodies. The latter could easily fall into the pop category (though metal bands have strong melodies too--see heavy, doom, and power metal), but the former? It's hard to say. I like Dream Theater's music best when I can emotionally respond to it, and that can apply to anything they write. For example, if I had to rank my top 5 songs, they'd probably be:
Innocence Faded, Learning to Live, The Spirit Carries On, Lifting Shadows off a Dream, A Change of Seasons
How could I classify that? I don't even know if I could distill the core elements that make me like those songs. Good melodies, maybe, but there's more to it than that. There's also great lyrics, great atmosphere, and just a general power to it all that's intangible.
I listen to a lot of prog, and maybe this is something prog bands do best. I'll consider voting for that.