I'm not usually someone that's hugely interested in speculation as I tend to be more focused on the here and now, even more so in the case of WoW in its current state - there are still way, way too many things to do before I could even think about getting bored with it.
However, there is one Cataclysm dungeon boss that caused my thoughts to go down an unusual path the other day: Earthrager Ptah in Halls of Origination. He's a pretty simple boss, tank and spank with some stuff that you're not supposed to stand in as well as a brief add phase, but the interesting thing about him is something else: on the terrace just below the boss, a couple of camels are lumbering around and waiting for you to mount them. You can then do the entire boss fight on the back of a camel. It's not a vehicle in the usual sense, as you still use your own abilities as normal, but it does give you the benefit of slightly faster run speed, and more importantly for ranged classes, the ability to cast spells on the run (since it's not you that's running, it's the camel). If you can do this on heroic without allowing Ptah to knock you off your camel, you get a little achievement.
Now, there's no real reason to put too much thought into that; you can just look at it as a silly little vehicle gimmick fight, but I thought it was interesting that this was the closest thing to true mounted combat that I can remember ever encountering in WoW. I vaguely recall that certain bombing quests in BC and vehicle fights in WOTLK were advertised as a sort of mounted combat initially, but in the end neither were really about using your character's own abilities while mounted so that felt a bit like false advertising.
Being able to fight while mounted is just one of those things... I've never heard people clamour for it en masse (unlike for player housing for example) but I think most would agree that the idea of being able to fight while mounted in general is pretty cool. Just imagine a paladin actually charging into battle on her charger, or a hunter pelting enemies with arrows while swiftly darting in and out of range on her mount. And that's not even taking into account flying mounts...
From a game balancing point of view it would be a veritable nightmare of course. In PvE, players would get up to all kinds of new kiting hijinks with the ability to fight while running at mount speed. In PvP, casters would suddenly be insane, being able to cast their most powerful spells while on the run on their mount. There would have to be abilities that guarantee to knock you off to counter this and melee would probably need to be buffed in general.
You could also ask: should druids be allowed to fight while mounted? At the moment the camel vehicles can be ridden while shapeshifted (click on the achievement link above to see a silly screenshot of a bear doing so), but while it's balanced and certainly acceptable for that one fight, on the whole the idea of a bear riding a camel is pretty ridiculous. Hell, seeing rockets strapped to bear butts in ICC looked less weird.
Lots and lots of issues to consider... but it would be cool, and since the developers have been emphasising the importance of the coolness factor a lot lately, I can't help but wonder whether they aren't considering mounted combat as new feature for a future expansion too. This little camel fight might just be a test drive, and I wouldn't be surprised if we saw a full-fledged raid encounter using similar mechanics before the end of this expansion. And then, who knows?