I think the song is very "complete", I think thatīs why so many people consider it the quintessential DT song. It doesnīt lack anything from any perspective you want to look at it. First, it perfectly blends the elements of progressive rock and metal, so people from different backgrounds can enjoy it. Second, it has great vocal melodies, so it has an element of catchiness that is important. The mood of the song also adds to its value, itīs not either happy or sad, but it definitely brings some kind of emotion to the listener, maybe pure excitement ("I want to hear more!!!").
The instrumental parts are very well constructed: the crazy middle section stands by itself of course, but if you pay attention at other parts such as the intro, the brief transitions between verses and the finale, youīll notice that the ideas are developed in a very controlled way there, instead of going for crazy fireworks, the sections serve the purpose of building and releasing tension.
And of course if you appreciate playing skills, the guys are at the top of their game in this song. Itīs hard to imagine how any world-class player in the world could improve any of the parts that 4 young musicians put together.