Night Elves are indeed evolved from dark trolls
Tauren do predate the titans (even though the recent ask CDev implied they didn't)
Pandaren have a society older than that of the night elves
Curse of Flesh IS what turned Vrykul into humans
The orcs drove a race of giants to extinction to build their temples, but this race was not gronn
The Ettin are from Draenor and are a "missing link" in the evolution of ogres
The Magnataur are related to Cenarius (what, did he get it on with an elephant? Cenarius seems to be a chubby chaser)
EDIT:-The who corrupted Sargeras is finally answered. The first demons that Sargers fought were dreadlords, infernals, doomguards, and what looked like a male succubus.
-Grell were created by Assenia (Brann says that she believes so, but can't remember much about creating them)
-He believes Kaja'mite might have sinister origins. Specifically it says "Need tae research that ore. Somethin's nae right about it."
-Kobolods are actually an evolved form of trogg.
-There are no records of frost dwarves in Ulduar, so Brann speculates they emerged from Uldaman with the regular dwarfs and then migrated to Northrend.
-Earth Giants apparently were turned into Gronn (perhaps via the curse of flesh) who then turned into ogres and ettin.
-Pandaren claim to be older than furblogs and are not related to them. Brann claims that he he learned this from Chen who's "puns are infectious".
-Brann believes all non titan races except trolls and their descendants are the offspring of various ancients. Tauren came from a "bull ancient", gorlocs from a a "frog ancient" (they later evolved into murlocs), Tuskarr from a "walrus ancient", harpies from Aviana, ect.
-Northrend is still crawling with iron dwarves, giants and vrykul. Thorim and Mimiron are trying to "fix 'em".
-Dragonkin are mortal races turned into half dragons by the flights. The process isn't voluntary, dragons just seem to abduct people and forcibly transform them. This is how there are so many black dragonspawn.
-The titans created five types of giants: Fire, Ice, Sea, Storm and Stone. They've all been affected by the curse of flesh, but not to the same degree as the mortal races.
-The absence of the Shadowtooth dark troll clan is explained. "Twilight's Hammer wiped 'em out, it seems. Waitin' fer the mess in Hyjal tae sort itself out before checkin' those caves."
-The Drakkari tribe is all but extinct. "Tryin' tae eat yer gods will do that."