I'm almost exclusively a cav player, but you'd be hard pressed to say the few comments I've made on balance issues in the past were anything but fair. I'm not interested in giving cav an unbalanced advantage, and I don't want to make other classes weak by comparison for some personal advantage. /disclaimer
First of all, when judging how powerful cav are, lets not even bother to count kills against players who wander around alone in the open not paying attention, or standing out in the open shooting their bow or xbow. The power of cav has nothing to do with it, and they will get killed as long as cav can move and attack. Likewise, getting attacked in the back isn't something that is related to cav power, unless you are talking about improving the sound in the game, which is a fine change if it is possible. But you can't possibly expect to nerf cav to the point where you won't be attacked in the back while you fight some infantry in front of you, at least not unless you plan to make cav so useless that nobody plays as cav...
Cav speed seems to be near minimum. Remember, much of what you see is the fastest horse (courser) with max heirloom and high riding skill. At low levels with the slower horses (Sumpter, steppe), an infantry can actually block the attack then slash the horse before it completes the pass, especially if the terrain is anything but perfectly flat. The lance many times will even glance off heavily armored foes at the low max speed of the slower horses. Even successful couch attacks, if you can get enough speed to do them, will not instantly kill armored foes. If you slow down the rest of the horses to these levels, it's going to be pretty ridiculous.
Cav maneuver could have room to be reduced without utterly breaking cav, but I don't see the need. An heirloomed courser with high riding still has a high turning radius at high speeds and must slow down a lot to make 90 turns into even into rather large corridors. At low speeds, they are still making passes (going in from one direction, exiting to another) rather than bouncing in and out from mostly the same direction like high maneuver cav can sort of do at slow speeds. For the high maneuver cav, that's all they have got going for them. They are much weaker and easily taken down by arrows (often making them a liability that will get you dehorsed in the middle of enemy territory), and less deadly because of less speed bonuses (and the weak lances depend heavily on speed to be effective).
Where I agree with "nerfing" is with the cav's ability to block attacks with a shield or weapon. The blocks extend too low, blocking some attacks that are clearly aimed at the horse and not the rider (or just the rider's legs). If it is impossible to change the hitbox size for blocking on horseback, maybe it is at least possible to raise the whole thing higher into the air when on horseback?
All in all, cav aren't doing anything that is overpowered to get kills. If they are doing well in a particular round, it's due much more to the infantry's failure to defend properly. It's kinda rare to see a pikeman defending a bunch of allies. Awlpikes do help a bit, but they aren't really meant for cav defense and won't protect a large swath of allies anyway. And then when there actually is a decent effort to fight together and have someone on anti-cav duty, there's still probably going to be some allies that only focus on their next kill and don't pay attention to their positioning.
If I get dehorsed and my team is still evenly matched, I feel quite comfortable using just my slow heavy lance to defend allies from cav. If I find a long spear or similar that has such a significant length, speed, and damage advantage over any cav, then any allies near me should be pretty darn safe from cav. If you guys have watched Walt_F4 play, he defends quite a large swath of allies from cav, either discouraging their charges or killing the ones that do charge, and he still manages to aggressively contribute to the fight against infantry at the same time. When he's not actively fighting infantry or protecting a particular group of engaged allied infantry, he's probably in a good spot that anticipates the path that cav will be using. For example, you know cav are going to run along the edge of a group of your allies and take a swipe... Walt is usually waiting in the spot just beyond that (or around the corner in city maps) where the horse has to travel on the follow-through of the charge.
Stick together, protect your allies, and cav are mostly target practice.
