Rogue, Priest, Warrior, Hunter, Mage.
Or in D&D terms
Fighter, Priest, Ranger, Mage Thief.
These are core classes, they should be allowed for every race .
Paladin, Warlock, Death Knight, Monk, Shamen, and Druid are specialty classes as such having Racial Limits is fine.
Paladin, Druid, Shamen, and Monk are specialized priests. In classic D&D they were limited to specific Races.
Paladin = Priest fighter, always Human
Druid = Nature Priest, animal focused.
Shamen = Nature Priest, Element focused.
Monk = Defensive Priest, Light melee focused, usually Human.
Now some may balk at the use of D&D in this situation, but Metzen and most of the Blizzard Developers play AD&D, and they base a lot of the game on D&D.
The reality is the Gnome Ranger is a bit of a D&D Trope.
Because of the racial bonuses and size defense bonus many people playing rangers and thieves would pick gnomes and halflings when building a D&D character.
Now WOW has changed the style of Gnomes a bit, the lore has made gnomes the leader of steam punk style in a fantasy world.
Which is fine, Gnome Hunters will be a little steam punk and technology driven.
The imagery is actually fairly common in Fiction.
The idea of the tiny woman with the giant gun, is what gnome hunters are like.
I think the reason so many, mostly horde players hate the idea of Gnome hunters is they realize how epic it really would be.
Just think You could make Seras Victoria.