You know, I never thought about that!
Here's another one.
If Humans can be paladins and priests, why are the undead only allowed to be priests?

I really would like to see undead paladins.

Because priests don't draw their powers from a specific place like paladins. Paladins draw power from their worship of the light and only light following races can become paladins. Priests on the other hand draw power from the deity they worship, not the light specifically. That's why there was all the QQing about tauren paladins since they currently have no contact with the light, worshipping the earthmother instead. It would be feasible within current lore for a tauren priest that draws its power from the earthmother, but not for a tauren to be a paladin as there are no light worshipping tauren in the game (that i'm aware of).

Personally I question each new race/class combination. I reserve my final criticism until I play them when I fully expect that Blizzard will work in explanations. If I were to guess why worgen can be druids, I'd say maybe they found nature in their seclusion. Maybe as ware-wolf type people they've learned to worship the moon?

Ancient legends tell of the worgen druids who were once very important members of the cenarion circle, but they had all but disappeared, off to another dimension (hint hint Emerald Dream) off to fight the lords of the emerald flame (hint hint emerald nightmare/burning legion).

Worgens = Scythe of Elune = Elune = Druids.

Makes perfect sense, actually. You should blow raspberries at yourself for missing the obvious connections.

Except humans that are infected and become worgen aren't really worgen, genetically or culturally speaking, so would have no natural connection to moon worship or druidism.

"Records indicate that the worgen existed for a time in Kalimdor. In fact, more recent evidence suggests that their true origin might have a connection to the night elves and a secretive druidic order from Kalimdor's distant past. Yet until new evidence comes to light, this information remains speculative."

If you assume the curse affected individuals on a mental level some citizens of Gilneas could have went down the path of Druidism like those who infected them. Then they would just teach others the same things and suddenly you have Worgen Druids as a normal class among the race.

Just to note, there is a already an Undead Forsaken hunter in the game. Nathanos Blightcaller, anyone?

The one combo I think is missing is Draenei Warlock. It's pretty much analogous to Night Elf Mage. I'd love to roleplay a reformed Eredar.