It's also possible that Tauren are uplifted cows, and they're 'pure' because they're young enough not to have been around when the Old Gods were unfettered.

I don't think the Tauren are "half" anything though. They're certainly not minotaurs like you may be thinking of from Greek Mythology. There's nothing human about the Tauren, except for their ability to walk upright on hind legs and speak. The Minotaur had the body of a man and the head of a bull. Tauren have fur, tails, hooves.....Tauren, through and through.

The biggest thing to me that separates Tauren from Night Elves is the Tauren penchant for Shamanism. I think as humans we get caught up in trying to humanize everything we see, so if we see something that vaguely resembles a man we try to imagine how a man might have turned into that thing at one point. Thus in discerning the "less human" humanoids of Azeroth we have a tendency to fall back on Druidism. But the Tauren's connection to the earth and the elements seems as strong as the Nelves' connection to the moon, imo.

I'll be surprised (and a bit disappointed, mind you) if we find out later that some Night Elf druid once turned into a Tauren and that's why we have Tauren now. Bliz has told that story already, and it's why we have worgen.

So Tauren would be mutated trolls? ;)

In their attempts to be closer to the earth, the night elves engaged in a forbidden transformation... cows. To the shock and horror of the populace these druidic cows began to... graze. Turned away by their noxious emissions, the remaining night elves banished their bovine bretheren to... that grassy area way over there.

@jbg lol nice

Always remember that (in the real world) "humans" are merely evolved animals as well. Just because primates have the best immediate chance to develop sentience and society doesn't mean other mammal families (or species beyond that) may never evolve into something sentient that is nothing like "humans".
Add Warcraft's magic to the mix, and you have several species at the sentient level. Just because gnolls, kobolds (rat-like in Warcraft) and quilboar look very non-"humanoid" doesn't mean they're "animals".