I really can't think of any major disappointments. And I have to work a bit to think of minor ones, although I can name a few. FII was definitely a disappointment at first. But after awhile, I came to really appreciate most of the songs.
I was a bit disappointed by the fact that DT cut their touring schedule a bit short and never really did a proper U.S. headline tour for Black Clouds.
I was also a bit let down by the last two 12SS installments. With TROAE, they appeared to realize that the songs could benefit quite a bit by being more concise. Then they went the other direction with the last two, which both contained some definite gold, but feel like they far overstay their welcome. The Shattered Fortress, in a way, exceeded expectations in that it took some previous themes and riffs that were already quite good and really elevated them. But at the same time, there were just too many reprises and difficult transitions for it to feel as cohesive as I thought it should.
Again, in the grand scheme of things, these are pretty minor.