I have a mounted combat specialist character. The problems I see with mounted combat are 2:
1) Overrrun and Trample:
The erratta nerfed overrun so that you couldn't do it as part of a charge, which absolutely killed Trample dead. It was a marginal feat anyway, and without being able to Trample on the charge, it's almost completely useless. The reason they changed Overrun was to stop people charging through their allies, which makes a certain sense to me, but the proper fix was to say that an overrun attempt was a standard action whether your terget tried to avoid it or not, rather than to make it impossible. Both ways you can't effectively charge past your friends and attack, but my way you can still try to overrun enemies, as is only sensible. Overrunning on a charge makes so much sense that before the erratta you actually had a bonus to it.
However, they are unlikely to change the erratta back, as they'd have to admit their mistake. The bandaid fix would be to add text to Trample allowing it as part of a charge. You still can't use it on your friends to bypass them, as avoidance isn't an option with Trample.
2) Ride-By Attack and Charging:
As others have pointed out, you can't ride past someone with ride-by attack, because you have to charge, and thus you have to move straight towards them. Yet, you can't overrun them, because you are charging. And while the feat avoids AoO from movement, it still doesn't allow movement through an opponent's area, which requires either 3+ sizes difference (not likely) or a Tumble check DC 25. Not many mounts can pull off the latter. Basically, Ride-By Attack also needs additional text that clarifies exactly how it works, as currently on a standard horse (no tumble ranks) it's not possible. Again: Ride-By Attack requires completely illegal actions, and can't work. It requires a fix. At least Trample can be used, if only without a charge, Ride-By Attack simply can't be used at all by 99.999% of characters who have it.
I can't emphasize that enough.
It needs to either allow movement through the target's square, or (more preferably) an exception to the charging rules to allow non-straight-on charges.
Obviously, when the charging and overrunning rules were rewritten for 3.5 and then the erratta, nobody actually looked at the mounted feats. They are broken, and not in the good way.
--Seule