I play a Draenei Paladin and our standard Paladin mount is the giant Elekk. There's a huge disadvantage playing a Draenei when compared to (for example) a Human when it comes to using the Divine Steed ability in tight areas (mostly indoors or at tunnel entrances or even in tunnels themselves, such as in Warsong Gulch), because the big Elekk simply doesn't fit through most things. The human mount fits through most things quite easily. In certain situations, this could cause trouble.
I think the same applies to Taurens vs Blood Elves on the Horde side. I would assume a Kodo also doesn't fit through everything the Blood Elf mount can fit through.
Am I wrong to consider this a legitimate concern? I just don't think certain races should have an advantage over other races that play the same class. I know it's only one ability, but still... it's quite an important ability.