I'm pretty sure the majority of class-combos we're getting with Cataclysm were previously restricted due to gameplay decisions, not lore decisions. For instance, Blood Elf Warriors were available at one point during the BC Beta (I played as one at the first Blizzcon), but were removed sometime after - Blizzard explained that they had to both A) keep a reign on the number of available race/class combos and B) make interesting class choices available. If I remember right, I think the class decisions came down to cutting one of the following: Warrior, Hunter, Rogue. The right decision seems pretty obvious.

I find too many people see race/class combos as what is POSSIBLE, as if classes have some highly exclusive list of requirements an entire race must meet before being "allowed in the club". Think of existing race/class combos in the following way: you can play as whatever classes a specific race has a strong interest in. It's not that Gnome Priests are *impossible*, it's just that Gnomes don't (or rather, didn't) have a very strong interest in pursuing such a career.

Same goes with Human Hunters.....trying to tell me that a Human cannot pick up a bow and train a dog because they don't have the proper ID and registration is flat-out stupid. Undead Hunters are only a bit more of a stretch, but there's plenty of diseased/corrupted-but-not-undead animals for them to pick from. I think it's safe to assume that allowing them to train all animals is a necessary gameplay decision, which is the kind of decision Blizzard has to make all the time.