These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Awesome Music: "The Obsessive Devotion," "Fools of Damnation," and "Martyr of the Free Word."
The Classical Conspiracy. All 144 minutes of it.
Pretty much any of their title tracks and/or grand finales, at least within the confines of their respective albums. Comparing The Phantom Agony's title track to, say, "Kingdom of Heaven" (either I/II) or "Serenade of Self-Destruction" might be a little unfair given that TPA was them just getting started, but when you hit the last song of an Epica album, you know you're in for a big finish and a hell of a sendoff.
Creative Differences: The reason Mark left After Forever and the reason Jeroen left Epica. Averted with Yves' departure however — he left the band to pursue another career opportunity that he couldn't fully utilise while part of Epica. Both he and the band posted public notes on Facebook thanking and wishing each other luck with their future endeavours.
Epic Riff: "The Obsessive Devotion," "Guilty Demeanor," "Internal Warfare," "Fools of Damnation," "Monopoly on Truth," and many more.
Spiritual Successor: The Quantum Enigma can be considered this in relation to Design Your Universe. The alternative title for the title track is "Kingdom of Heaven II!"
Uncanny Valley: The artwork for Requiem for The Indifferent embraces this trope...
Younger Than They Look: Simone in the first two albums — she was still eighteen when The Phantom Agony was released and had just turned twenty by the time Consign to Oblivion was released. Nowadays she's in the process of inverting this trope.