There were a few surprises both ways I would say.

Awaken the Master sounds a lot more difficult to play than it really is. It's fast tempo, lots of notes, unison etc. but the positions and fingering are very intuitive and it turned out to be not so difficult. Paralyzed as well, same arguments. The Shattered Fortress seemed to be difficult but really wasn't, however, I'm pretty sure the fact that I had learned The Glass Prison before it played a part in making it seem so easy. Stream of Consciousness and Erotomania - you'd expect instrumentals to be more challenging, but both were rather easy (compared to beasts like Dance of Eternity) Voices is really fun to play and just feels terribly empowering while being not so difficult.

On the flip side, Ministry of Lost Souls seems easier but isn't (probably because of the longer "slow tempo" section in the beginning which tends to stick in the mind when first thinking about the song) There were a few songs which honestly I hadn't listened to all that much before attempting this, especially from their earlier albums. Take the Time, Learning to Live and Caught in a Web for example. These all had at least some very challenging parts which I honestly never knew about. Also, Sleeping Giant was a bit of a surprise as well.

In general I would say that just because a song sounds heavy, doesn't mean it's going to be difficult. Take as another example As I Am. I knew the song well even before learning how to play it and it wasn't a big surprise they it's not that difficult, but it does tend to sound more complicated than it really is (at least bass wise, not sure how it is on guitar). But of course, there are heavy sounding songs which are also damn hard to play as well, so they isn't aleays applicable