Nelf Mage, Holy Cow

There's an awful lot of debate and anxiety over this issue, mostly made up by a bunch of QQ'ers who think they know more about the lore than Blizzard does. I doubt this will set the record straight, but I'll make an attempt anyway.

Night Elf Mage. Make sense? Yes.
Night elves were perhaps among the first and greatest mages before the War of Ancients; Azshara and her High Borne were probably the greatest ever. They had the Well of Eternity, and even though it's gone it still has an impact on the world ten thousand years later. Night elves got the ball rolling as far as mages go. After the War of Ancients the druids decided to ban arcane magic use, which they were pretty adament about. They exiled the rogue highborne who would later become High Elves, they stuck Illidan under a tree for 10K years. All of that though was an effort to keep the Burning Legion from entering the world again.

Well that threat has kind of been dealt with, at least for the time being. The Burning Legion came close to invading the world again, but they got beat back, and now they're licking their wounds. During Cataclysm, other threats are going to be presented to Azeroth: Deathwing, Azshara, the return of certain Elemental Lords like Ragnaros, and, of course, the remaining Old Gods. None of which really give a shit if you use arcane magic or not.

In comes the few remaining high borne elves who've been living in Dire Maul all these years, who still use arcane magic, offering to help. Malfurion is not stupid and knows that any edge can help with the current threats, so he sees the inclusion of mages back into Night Elf society as a logical decision.

There you go! Lore for Night Elf mages!

As for Tauren Paladin, yes I do agree that there is no history to support them embracing the light but there's nothing saying that they can't be paladins either. Personally I believe that the tauren are the most benevolant race on Azeroth, even more so than the draenei, who have a smudge of judgementalism about them. Because of that, Holy Cows do make sense as far as I'm concerned.

I hope that helps.