I think we can all agree that horses are very delicate? They die from three to six arrow hits, which isn't alot in cRPG, really.
And I think we can all agree the horses are really fast an maneuverable. Except the plate ones, which just sorta suck.
So we have delicate horses, which are fast and maneuverable; they rely on speed and suprise (they're hard to hear) to get their kills. I find I get the most kills when the infantry gaggle-fuck that is my team has advanced to contact with the enemy's gaggle-fuck. In such a situation, both gaggle-fucks are distracted by eachother, and that inclues both team's archers (which are either dueling eachother, or sniping infantry). A cavalry player need only stab distracted baddies along the flanks and rear of the enemy's gaggle-fuck.
The best way to kill the horse is to counter its speed and sneakiness. Turn up your volume, stay aware of your surroundings, and use the natural horse killing attack: the poke. Any and all ranged attacks counter cavalry, obviously. The horseman is unable to defend himself against projectile weapons, and instead must hide somewhere or maneuver around wildly to avoid getting hit--charging at an archer works, but in the process the horse will get hit at least once (possibly in it's large head). If a cavalry player is caught in the open and is fired upon by multiple archers, his or her ability to suprise their opponents is worth nothing.