WoWHead has been doing plenty of data-mining as well, and one of the cool things they've uncovered is the various voice files for Demon Hunters. Unlike Death Knights, who simply got the normal voices, but modulated and given a kind of echoey quality, Demon Hunters (probably thanks to the limited racial options) have all-new voices, with Demon Hunter-specific things to say.
Likewise, it looks like Demon Hunters will actually have appropriate error messages for lacking Fury or Pain. It never made sense to me that Worgen, Goblins, and Pandaren don't have emotes for Focus or Runes.
The male Night Elf definitely channels the Illidan voice. The female Night Elf goes a little growly. The male Blood Elf totally sounds like some kind of ultra-macho action hero, and the female Blood Elf has something of a diva thing going on. I love them all.
There are a metric ton of sound files (so much that WoWHead had to split them into three different pages.)
There are also a ton of new sound files for various spells, attacks, and melee strikes. They are trying to make melee feel a bit less "floaty," and I think one of the ways they're trying to do this is to make more distinct, impactful sounds (animations will also be getting work.)
This is definitely some real minutia, but it's fun to listen through if you just want to obsess over Legion while we wait for the Beta proper to launch.