I think Progressive can mean different things to different people. The problem with the genre is that at the peak of Progressive Rock, it was something new and revolutionary, and some people took that as something progressive. But if progressive means new and experimental by definition, then every band that's ever created a new genre or subgenre is progressive at their inception. By that definition, Soundgarden were progressive in the late 80s, and Metallica were progressive in the early 80s. But they can't be called "Progressive Metal" or "Progressive Rock" bands.
If Progressive Metal is a genre, then it can't be something that's new and experimental, because then it would be a genre that's always changing and constantly undefined. By that definition, Images and Words is a progressive album, but any album that sounds like I&W, but is released thereafter, is no longer a progressive album. In which case, what exactly is it, then?
That's why I don't go for the whole, "Progressive music has to be experimental" crap. If that's the case, then it should just be called "Experimental Rock" or "Experimental Metal" shouldn't it?
Progressive, to me, is a self-contained statement. If a song progresses in itself, with time signature changes, evolving musical ideas and motifs, then it's progressive. And by that definition, every DT album fits the description perfectly.