At the time, I was fairly derisive of Halion. What was the point of another small raid when we already had ICC? I remember doing heroic Halion attempts in July and feeling like the whole thing was a complete waste of time and a sidetrack from ICC. But now that we've had the same experience in Cataclysm of a long time in our end raid, and this time no intermediate raiding to tide us over until Mists of Pandaria, I'm rethinking my position.
Halion served two purposes. First, he introduced us to new mechanics we'd bee seeing again in Cataclysm. Both Valiona and Theralion and later Ultraxion used elements of the Halion encounter's mechanics. But second and more importantly, he served as a bridge between the ICC fights, with their Scourge, undeath and plague motifs and the coming expansion's introduction of Deathwing and his Twilight's Hammer cult minions. A third but related purpose was to give us something to do that wasn't ICC after six months in the place.
So now I wonder: Did Mists of Pandaria need an intermediate raid? Was it a missed opportunity that we didn't get one? And would it have made sense if we had?