One thing that has always been a "reality break" in 3.5 is the concept of using a 2 handed melee weapon off of horseback. As far as I know, there has never been a military unit that used a two handed melee weapon from horseback for the simple reason that you would cut off your horse's head and then fall ignominiously out of the saddle because you had no balance.
Naginata, used by Japanese bushi from horseback. It's a two handed polearm, essentially a cutting blade mounted on a roughly 5 foot pole. They used to ride through the infantry slashing to either side. Never managed to cut the heads off their horses. The Nagamaki, a shorter heavier blade was also used in a similar manner. Chinese made similar usages of slashing pole arms. Bows are, of course, routinely wielded from horseback as well. It simply requires (well, not simply it takes real skill) the rider to control the mount with knee pressure. And be coordianted enough to wield the weapon effectively. Something the medieval western knight was quite accomplished at. There are western examples if you care to knock around and look. The widespread use of the kite and heater shields in the west limited the visibility of the two handed weapon style from horseback. Some weapons might be more difficult to use (or even impossible) from horseback. I would think it's covered by the mounted combat feat, although you could propose a second feat be required to use two handed melee weapons from horseback. As for the greatsword, if you've ever seen one wielded by an expert (say Tower of London Armoury reenacter) it's impressive for the speed and power -- not what you normally think of when you consider a greatsword. Hard to say how well it would adapt to use on horseback although I suspect it could be done with reasonable success.
I quote from Wikipedia: "During the Gempei War (1180-1185), in which the Taira clan was pitted against Minamoto no Yoritomo of the Minamoto clan, the naginata rose to a position of particularly high esteem. Cavalry battles had become more important by this time, and the naginata proved excellent at dismounting cavalry and disabling riders. The widespread adoption of the naginata as a battlefield weapon forced the introduction of sune-ate (shin guards) as a part of Japanese armor. The rise of importance for the naginata can be seen as being mirrored by the European pike, another long pole weapon employed against mounted warriors."
I see no mention of this weapon used FROM horseback, rather used as a polearm AGAINST cavalry.
Also from Wikipedia about the Nagamaki "The nagamaki is designed for large sweeping and slicing strokes. It also works as a spear. Traditionally, it was used as infantry weapon. Warriors used the weapon against horsemen. Still, it required more time and materials to create a nagamaki than spears or naginatas, this is why it was not so widely spread. The closest exemplar of real nagamaki you can see today is nagamaki-naoshi. It appears to be like a long katana-shaped halberd, but straighter and thinner, with a very long tsuka. In contrast to it naginata is shorter, wider and more curved to the tip."
Once again, an infantry weapon used in two hands AGAINST...
From "Secrets of the Samurai" (Ratti, Westbrook page 202) "The long sword (nodachi) like the heavy sword used by European mounted knights, was also particularly adopted to equerstrian swordmanship and used according to precise techniques of kenjutsu against a peer on horseback or against hordes of foot soldiers..." The nodachi is a two handed sword. Specific mention is made in the book of the use of the knees to control the horse allowing all martial arts to be used from horseback -- including unarmed techniques. I'll track down the bit about the naginata and nagimaki later (a different book). I have a graduation to attend in about 20 minutes. This work is probably the best survey of the japanese feudal martial arts. A good read. Later, have to run, Ive got students to graduate.
More two handed goodness... if you're interested Stephen Turnbull's "Warriors of Medieval Japan" has a period painting (the action is in 1185 western calender) of the warrior monk Benkei wielding a naginata on horseback in combat. Page 15 in the introduction. He is controlling his horse with his knees and has the naginata overhead, in two hands, with the blade forward, cutting edge up. There are more examples if you want to find them. The lack of use of the sheild in japan makes it easy to find examples of two handed weapon use on horseback. It would be harder to find evidence for European use, but if two hands are available and the skill to control the horse without using the hands is there, as it was, I'm sure two hands were used with a weapon.