How you handle initiative for two creatures acting simultaneously is a biggie. Ditto once readied actions come into play.
How a charge happens can be confusing. RAW, I think the mount charges and the rider can choose to also use a charge action at the same time, or do something else with his actions, but he takes the -2 AC penalty from charging regardless. But the rules are VERY unclear.
Adjudicating Ride-by-Attack's RAW (basically impossible to use unless you kill the target on the attack) vs. RAI (charging at them from a side and riding past, jousting style, which the charging rules which require the most direct line path do not allow for) is also pretty big.
If the character takes trick riding, whether the first use of Mounted Combat to negate a hit or the second is what costs you your immediate action is yet another question.
A later issue, if Mounted Skirmisher lets you "pounce" with a shorter-than-the-maximum-distance-allowed charge, is another debate. I personally think that one's just silly and people are upset that the high level mounted guy can charge and full attack and just don't want it to be true, but whatever. It's a debate.
One of the things we ran into was whether mounts provided soft cover or not, since the rider is supposed to be in whichever portion of the mount's space was most advantageous.
Since you're playing a cavalier, your mount can become intelligent. How an intelligent mount is treated should be discussed with the GM. Who determines its actions was a sticking point with some of my players. (It's technically an NPC and therefore might have it's actions decided by the GM. It's also a class feature and therefore should be dictated by the player.)
Figuring out who takes Trample is important, if you plan to use it.
Charging, ultimately, is the big issue. Aside from the ones StreamOfTheSky mentioned, there's also using overrun on a charge and how that interacts with the rider.
Is there any consensus on whether a rider with a reach weapon can attack when charging with a mount who does not have reach? Can the mount attack as well?
Is there any consensus on whether a rider with a reach weapon can attack when charging with a mount who does not have reach? Can the mount attack as well?
I am wondering how this works too. I have a cavalier on my campaign (Kingmaker)and I have been allowing him to charge, use lance, then letting his war trained horse make an attack too. Should the horse get one attack or three? (hoof, hoof, bite).
You can find nearly all the mounted combat rules right here, in the Combat chapter. The only thing I have to offer about exotic mounts is to talk with your GM, and to make sure how differing mounts might effect the cost and weight of barding.
StreamOfTheSky, mount and rider act on the same initiative. Not much of a biggie. Easy to understand. Mounted charging also isn't confusing (the intent is quite clear), people just like to over think the English language, pick it apart, and then argue on the internet (it's its own form of entertainment these days). Ride-By Attack IS impossible as written, but I think the intent is pretty clear at least. I don't know about Trick Riding and its interactions with immediate actions. Mounted Skirmisher's with charges is indeed another mystery.
Mauril, mounts don't provide cover unless you make a ride check as an immediate action to use them as cover. Trample is a rider feat, not a mount feat.
Howie23, here are the links to the v3.5 mounted combat rules article you were referring to: All About Mounts Article One, Article Two, Article Three, Article Four, Article Five.
Muninn, a rider can charge with a reach weapon. The mount cannot also attack unless it too has reach.