Copied from the discussion on shields.

I think the main reason why two-handed weapons see so little use compared to shield + onehander is mainly that the typical damage output differs so little.

For example, compare a regular two-handed sword to a regular iron sword. The base damage of the two hander is better, but that difference is watered down because the strength bonus is the same for both. Add in further bonuses like weapon skill, and the real difference in damage is quite small.

I think it would be fair to increase the strength bonus to damage when using a two handed weapon. After all, you can swing with both arms in full force, rather than with the strength of one arm.