Personally, I have heard several sections that, to my ears, actually reference past DT songs/albums. A couple that immediately jump to mind are:
-the intro to Heaven's Cove (On The Backs Of Angels)
-a vocal bit in Moment Of Betrayal (The Great Debate)
-the Arhys/Daryus back and forth in The Path That Divides (similar to verses in This Dying Soul)
- the last minute or so of My Last Farewell (The Ministry of Lost Souls)
- the NOMAC tracks (the transition between Panic Attack and Never Enough)
I don't know if anyone else has heard these or if it's just me, and I'm sure there are more that I can't think of right now, but I think it's kind of cool that they are self-referencing a large chunk of their past. (I mean, I know that they have done this before but I don't think I've heard references to so many different albums in one album before.)
Edit: How could I forget- the into to Dystopian Overture is extremely similar to A Nightmare To Remember.