i heard any race can be a DK

Tauren's backstory doesn't fit with DK's at all. They are more wandering spiritualists, if any race limitations are on it I'd go with Orc-Undead (why wouldn't an Undead command other undead?) and Human-Dwarves (playing up the lineage that some DK's are fallen Pallies Dwarves can be the balancer race).

My logic in these is that DK is like a Lock/Warrior hybrid (like Pallies are Priest/War) and orcs, humans, undead are the races that can be locks (toss out BE so that there is a balance of racials on each side) and like I said, toss in Dwarves to fill in the fallen paladin angle (gnome DeathKnight? Plausable, but meh) and keep the sides balanced with two races per faction.

Looking at the history I don't see Tauren or Night Elves (both more naturalistic races) being Death Knights. Draenai as well since they are all against evil and whatnot.

I would not call a Death Knight a fallen Paladin (or Donkey Kong for that matter).  Arthas was the first Death Knight, corrupted by Frostmourne and slaying his ally Muradin Bronzebeard just before releasing it and weilding it.  Death Knights are necromancic knight, not necessarily a corrupted knight.  You don't necessarily have to be evil to command dark magic.

I think the best comparison to use would be a Shadow Knight from Everquest.  Shadowknights could command the dead, DoT and deal excellent melee damage.  I am hoping the Death Knight will be played similarly.  And I also hope that Horseback combat will be implemented into combat (and yes casters could cast on horseback if their horses are moving while they sit and cast).