While I would have no objection to a complete curve ball of an album, the band's usual approach of putting a more minor twist on their core sound with each new record is usually more than enough for me. While the changes don't always seem all that significant at first, if you compare albums with several years between them, you realize that DT have actually done a really nice job of trying out new things. I know quite a few metal heads who love Train of Thought and the self-titled but don't really enjoy Falling into Infinity or The Astonishing because they're simply not heavy enough.
In fairness to the band, I also think that the difference between DT12 and TA was quite drastic. On a one to five scale of "We're experimenting with our sound, maaaaaan" where a 1/5 is Ride the Lightning -> Master of Puppets and a 5/5 is Watershed -> Heritage, I'd say that DT12 -> TA was a solid 4/5. Between the shorter songs, lighter tone, cheesier lyrics, and increased acoustic playing, it feels like a very different record to me, at least.