Has anyone said ‘mounted combat’ yet?
Because, mounted combat.
So I'm helping a friend of mine create his own system (I keep trying to dissuade him, but he's adamant. Ah, youth) and we just had a discussion this point.
Him: I want there to be mounted combat specialists in my game!
Me: That's great, but what happens when you can't have or use your mount?
Him: Uh, well, that's a trade-off, then. Not everyone can use their abilities all the time.
Me: Sure, but, like, the situations where a spellcaster has nothing to do are potentially fewer and farther between than a warrior. And even if a warrior has to sit on his hands during say, a social encounter, if a fight breaks out at a social gala, he's golden, even if he has to improvise a weapon. The cavalryman, however, has the additional problem of not generally being allowed to have a horse while in a dungeon or building.
Him: and? People will know what they are getting into.
Me: if you're fine with that, ok, but I'll tell you right now, anyone who realizes the issues with that kind of specialization might just avoid it entirely, possibly making the whole point of having mounted combat abilities pretty pointless.
Even the times I've seen this solved (summoned mounts for Paladins, scaling mounts for Cavaliers), there are still going to be times when a good portion of your character is disabled. The Wheelman in Spycraft, for example, drives cars really well. So well in fact, that he's about the only one who should be engaging in vehicle chases. But since you're only having to drive like you're in downtown Beirut every so often, they compensate by being the off-tier "combat" class, being closer to the Soldier in effectiveness than anyone else*.
*Going off my admittedly fuzzy memories of Spycraft.