Either cav should be fucking hard to kill, and tankish, and able to counter ranged by tanking their shots, closing the distance, and fucking them up.
Or, cav can be ninja like, fast, and sneak up on ranged to fuck them up.
Or, OR, you can have both. Some horses tank, and some horses sneak up on people. This, I think, is how it should be done, because it'll allow for more flexibility. This, I think, is already being done, but could be done better. The courser and arabian horses are fast and can sneak up on players for gay kills. The plated horses, and the destrier, are heavily armored and have much hp, and can bum-rush players for gay kills.
I think, however, that the differences between the fast horses and the tank horses should be a bit greater and should be more exaggerated. The faster horses are, I think, fine atm (it's why I ride a courser). The heavy horses are a little too fast, and not quite as tough as I'd like them too be. They're also too expensive, at least if seen as another option for a cav player, rather than as simply "better" horses.