Smooth and WITCHCRAFT hit the nail on the penis.
From utop my OP horse, I am in a good position to observe the flow of battle. Usually, 2hers do best when skirmishing on the sides, when the line's break apart (more skirmishing), and as somebody to support the shielders and pikers (standing beside pikers, behind the shield-wall). Rushing behind the enemy line and spamming helps, too.
2h is also great for anti-cav. Pikers usually stall out enemy cav, but they rarely finish them off--you can, though, with ease. You can also lolstab cav on the flanks, or fuck them up as they make their strafing runs through your lines. Rarely do cav pay attention to anything but their intended victim.
Siege is totally different, and better for 2h imo.