Abuse of the rules?
A more powerful build is one that specializes in the lance; you get to use one hand if you want, do double damamge on a charge, and get fantastac reach with it. Combine that with Combat Reflexes and you have a mounted combat machine.
Comparatively, a two-handed weapon on a mount is relatively tame... you don't have the reach to use your Combat Relfexes, don't deal double damage on a charge, and don't have the option to use it one-handed.
"Ah," you might say, "but the two handed weapon can be used off of a mount as well as on! There lies the disadvantage with the Lance!"
What on earth is preventing you from using a lance while on the ground? The *only* difference between a lance and a longspear is in their special abilities: a spear can be set against a charge to deal double damage, and a lance deals double damage when charging from a mount. That, and the ability to use a lance one-handed while mounted, is *it*. For my money, the lance is the best reach weapon in the game because it can be used to such devastation on a mount, and isn't shabby off of one.
So... how is a mounted character "taking advantage" of the "hole in the rules" by wielding a two-handed weapon? Especially considering that this is a sub-optimal build?