Dream Theater have been 'ripping people off' for years - most bands do. The difference is that they used to just rip off interesting things.
For example, the intro to Erotomania was influnced/ripped off by a line in a Mahavishnu Orchestra improvisation. THAT is a cool thing to take notes from.
But new DT get's their material from the obvious: Muse, Pantera, Metallica, ho hum.
I don't mind well executed musical theft. Good artists borrow, great artists steal and all that. I just wish the guys in Dream Theater cared about making interesting music again, which I don't think they have for a very long time. But hey, they're all married, kids, mortgages and all that. Why should they take chances anymore?
And I don't hear much Comus in Opeth. Mike's biggest writing influence is probably Andy Latimer from Camel, as far as I can tell (though new Opeth is getting wild). Check out Camel's 'Never Let Go,' and compare that with Opeth's 'Benighted' if you want to have a nice giggle.