EDIT: Heart of Pandaria is now active as the beta login theme.
Let's try to get ahead of the end of the school day on this. Let's put the 20,000 "login music" threads in here instead.
For those of you who don't know, the likely login screen music was added in the latest patch. "Heart of Pandaria" is an 8-minute track on the front page.
I honestly feel this is the best login track yet. The first thing I noticed was it sounded like it was recorded live instead of just synthesized. The quiets were quieter, the instruments had more flow. Secondly, it fit the theme very well during the first part. After the traditional WoW opening line, it moved into some very traditional Pandaria music, interrupted then by conflicting tunes from Stormwind and Orgrimmar. Wonderfully matching the "war theme" with Pandaria "stuck in the cross-fire."
But the back half really did it for me, when the song turned into a clearly asian, yet still heroic theme. Almost like out of the war emerged new heroes.
But anyway, I could go on for a while. Post your feelings below, and if you see any of those 20,000 threads later, point them here.