The Sha of Fear is a terrible end-boss.
Having killed him on normal mode at least, I can say that the fight is.... okay. It's a bit more evocative, the symbolism used is appealing, the mechanics at least on the surface are engaging, but there is no follow-through on it. The fight begins exactly as it ends, it doesn't flow or change at any point.
The best fights, I think, tell a story. They begin in one place, end in another and along the way, they take you somewhere. Even if the story is cyclical in nature, it should vary enough to be worthy of the cycle and have enough depth worth telling. Tell Alysrazor for example, we were engaged with her before we ever pulled and though cyclical, every one of her phases is tied deeply in her rage and her elemental nature.
The Heroic Spine of Deathwing, on the other hand, is an example of a cyclical fight gone horribly wrong. The mechanics don't change as the fight progresses, they just become increasingly more frustrating to execute. It's purely a numbers game and there's nothing engaging about that. Sha of Fear is cut from the same cloth of uninteresting cycles.
We begin by standing on the platform. Tanks takes their place on the beacon, enveloped in a protective halo of light - evocative, if sentimental. The light guards everyone from the worst of the Sha's corruption while outside the beacon, we can see his twisted minions writhing about, and the corrupted Pandaren on their distant platforms. Then, we ourselves are contorted into Sha and sent to be killed by the Pandaren who we must defeat in turn to be returned to continue our fight.
And then that repeats, over and over again. Other than clearing the Crossbow men and the adds on the platform, this is a tank and spank fight.
Where is the flow? Where is the story? What do we learn from this encounter about the villain that we didn't already know?
Blizzard is capable of incredibly good storytelling in raids. We know this, from Ulduar and Karazhan cohesively, but also on a smaller scale. Consider the tight narrative of the Plagueworks. Or, if you want a single encounter, take Nefarian as an example in either incarnation or the Lich King. Each phase transition tells a story, it evokes a feeling, it progresses the arc of the character and shows us an aspect of that personality.
One of the problems here is that the Sha has no personality. By definition, it's a one-dimensional creature, and thus is incapable of presenting complexity as an individual. The only way to make the Sha personal, to give it gravitas and any sense of depth, is to imbue it with some purpose.
And that is exactly why Empress Shek'zeer in the Heart of Fear works brilliantly as the crowning achievement of this raid tier. She is the true end-boss in every way. Besides the fact that Heart of Fear as a whole is tuned perfectly (except maybe for Amber-shaper where Discipline priests are perhaps a bit over-powered) and the entire raid instance as a whole tells a story in addition to the individual bosses who have their own narratives,
Here, you can feel the Sha of Fear in all the rooms, you can smell him in the walls, see the corruption left behind and when you get to the Empress at last - she's mad with paranoia. We know, through the Klaxxi quest line how the ascendance of a new queen happens, and gnawing at that thought, that fear, the Sha has infiltrated her and made her this thing, crawling with shadows, overflowing with dark energies.
She begins in a rage, screaming stories of her own glory to deaf ears, and when she doesn't get the reception she wants, she summons her subjects to tear her audience down. Deprived of her coterie, she returns, deaf once again to her own doom, repeating her aggrandizing
song. When death at last is evident, she panics and the fear grips her with both hands, furious for survival, she fights like a cornered animal, raging for life, throwing everything she has at us until we grant her the only mercy we can.
That is a story. That is a narrative. That is what makes a raid boss interesting.
And that is why the Sha of Fear is a terrible end-boss.