So who are those demons on the thrones?

[marshmallow]

You know, during the black soulstone cinematic? Lots of speculation before the game came out, but it was totally ignored. No clues in the CE art book as far I can tell either.

Generic Azmodan underlings?

Since it's Leah having a freaky vision, maybe it's supposed to represent the trapped evils in the soul stone and whoever sits at the red throne can become the prime. Total WAG though.

[Saint Anger]

For some reason or another they made me think me of the Vizjerei (the mage clan who first brought demons into the world of Sanctuary). There is nothing to substantiate that though.

[raveharu]

Good question, but I think those are Azmodan's generals.

We'll probably see them in expansion since all the evils have been trapped in the black soul stone.

Remember that what has happened so far has been planned out by both Azmodan and Belial.

[GoldenredDragon]

And also planned out by Diablo ever since before the first Diablo episode.

G.R.D. (And if we believe in the "Wrath" video, even before very long ago !)

[Buen]

When you go to destroy the sin-Hearts you see that theese dudes are placed around the edges of the walkway.
I think the vision just shows the impending treat that Azmodan represents, and among other things, his 'underlings' that manage thoose sin hearts.

[Tahani]

They were in diablo 2 as well, when you go from river of flame -> chaos sanctuary like two rows on either side of you whose eyes light up and follow you