Tauren paladins are called Daywalkers, or SunWalkers, not taking their power from the light itself, but taking it from the idea of the Sun that spreads it's own light over the world.
Trolls are related to night elves, you should remember this, Trolls have been shapeshifting for years, just look at ZG and ZA, it's not that outlandish of an idea that some Trolls learned to control said power and turned into druids.
Gnomes were given Priest for the Class / Race combo ratio to balance out, and also do the the gnomes racial for inreased intelegence I do beleive, makes a gnome not that bad to think of for a caster class, the preist was just to spread out more of the healing classes for combos.
I honestly don't remember hearing about Human shamans... but hunters yes, make sense, look at hunters today, they have "pets" and hunt, so whats the problem there?
Dwarves were originally earthen that had control over the land to an extent, how hard is it for some dwarves to revert back to their origins and perfect it to where they can control more then just the earth.
Also you must remember, this is during the time of a Cataclysm, everyone is looking for ways to survive the now explosive evil that has tried to destroy the land, that means people are reaching out to new ideas to try to fight for survival, so really, execept for the undead hunter, I can see most of the situations, the undead hunters are just... umm.. wasn't there a WoW-comic explaining WHY undead weren't hunters? XD though I would guess it would be funny for a roleplayer, Roleplaying their pet stealing their arm constantly or something ^^;
A wise man once said, to never underestimate the potential of human stupidity.