Progressive Metal ((wikipedia, allmusic) is an interesting sub-genre.  At the risk of oversimplifying one of more diverse of the metal styles (and when have I shied from oversimplification on this site?), progressive metal is what you get when a bunch of art and music students set out to creating really shredding music.  Some of this music misses the mark metal-wise, in my opinion (I mean c'mon, Queensryche?  King's X?  Not bad music, but Metal?  More like Hard Rock)  Most Progressive Metal, however, does deliver when it comes to unique song structure, multiple time signatures and long, epic compositions.  Vocals often tends towards the operatic (not a strong selling point for me personally, but some like it), and the lyrical content is among the most diverse in Metal.  As I've mentioned above, some of this feels more to me like Hard Rock or Metal-influenced Pop than it does true Metal, but other acts (among them Anacrusis and Tool) can kick and thrash with the heaviest of them.