Frankly, for me it's more of an issue behind the roleplaying aspect of the game. Sure, 4th ed is fairly easy to re-skin a character or a class to be completely different and it is feasible to create a animal character such as a leopard by re-skinning the druid or creating an NPC follower template. The problem comes from how the roleplaying aspect will be affected.
I am from the school of thought that characters that are deep and have more details are fun to roleplay. Conversely, I think that the more exotic a character is, the more shallow he becomes.
Imagine for example, a human wizard compared to a Gnoll barbarian. The human wizard is definitely the more plain-looking character, but he can be fun to roleplay if details are added to him. Perhaps he is nihilistic, or is overly happy, or is magically insane. All these will imediately make the character more interesting to roleplay.
However, how can the Gnoll barbarian be roleplayed? He is more easily stereotype, he likes blood and killing and death. What does he like? Gnoll things. What does he think? He thinks like a Gnoll. What does he wear? A Gnollish outfit. Suddenly, because you wanted to create a really exotic character, he becomes so completely one-dimensional that he becomes boring.
Of course, the Gnoll can be made interesting, say if the Gnoll was raised as a pup by dwarven parents, or if he is trying to prove that Gnolls can be peaceful creatures. But it's far more easy to fall into the common Gnoll stereotype than to break from it. I would be very wary of characters trying to create exotic PCs simply because they want to and not for the roleplaying aspect of the game.
One of the reasons I enjoy the New World campaign so much is because the characters are all deep and break away from molds. The Orc leader is actually very intelligent, the lich seeks the greater good, the Kobold calls himself an engineer... few of the characters, whether PCs or NPCs, are so exotic that they never develop a personality of their own.
Playing a leopard that has no human intelligence and thinks like a leopard and acts like a leopard seems to me like a very boring character, created solely for the reason of being exotic, and I would actively discourage my players from playing it.