It's pretty damned Eurocentric to say that elves, dwarves, gnomes, and the like are just part of "human mythology". Hell, it's northern-Eurocentric to say so - there are no such creatures in Greek mythology. A setting which treated Greek mythological creatures the way D&D treats Germanic mythological creatures would have humans, satyrs, centaurs, dryads, et cetera as zero-LA options, perhaps.
(I say this despite the fact that humans are the heroes (or at least protagonists) of all Greek mythological tales that don't revolve around outright gods; northern European mythology didn't feature nonhuman adventurers as much as D&D does either.)
There's a hell of a lot more to human mythology than just elves and dwarves even if you don't look outside Europe.
As far as the original topic is concerned, the main problem with the traditional D&D races is just that - they're traditional. Like most traditions they can be rich and strike a deep chord in our imaginations, but again like most traditions they rarely do. Your average D&D gamer has played dozens if not hundreds of sessions with the same old tired Tolkienesque takes on the races, so it doesn't surprise me that we look for completely new (if often unimaginative) races to play rather than try to reinvent the same old ones that we've become burnt out on.
That said, when creative minds constrain themselves to a specific task they can succeed at it in unexpected ways - I would have sworn up and down that nothing Wizards of the Coast could put out would make me interested in elves, gnomes, and halflings, but Eberron proved me wrong. I still don't necessarily feel jazzed by everything the setting does with the races - for instance, dwarves are fairly bland - but it's still vastly more interesting to me than the standard tropes.
(The only other problem I have with Eberron's take on the standard races is that those members a given race who don't belong to one of the original cultures or to a dragonmarked house don't really have even their traditional associations. If you're an elf whose family has lived in Sharn for centuries, but you're not Aereni or Valenar and you're not in a dragonmarked house, what are you other than a human in drag? They exist - there are halfling crime organisations, though even they have ties to House Jorasco - but they don't have a hook in and of themselves. That's one thing I'd like to see in the future: more detail on how the nonhuman races live in the human-dominated lands when they're not just transplanted enclaves from other nations.)
I'm open to new races when they've got something original to offer. For instance, while I may never use them, I admire the design of the goliath race in Races of Stone because they have an interesting tribal culture of competitive cooperation and fun mechanical traits like their Powerful Build and skin markings. They offer something to hook the imagination whether you're focused on personality or abilities.