The only 2-handed weapons I have seen so far have been at most only a point or two higher in damage than a 1-handed weapon.
Why would I ever want a 2-handed weapon instead of two 1-handed ones? It seems like two one-handers will always be superior because you can have two sets of bonus effects.
You’re looking at DPS, not Damage, right?
Two Handers tend to be slower – you attack much less frequently – but hit much harder. This is particularly useful in combination with the way several channelled or high-cost skills work – you’ll conserve your resources more while doing comparable damage with a big heavy weapon, at the cost of defense, or possibly some extra damage. You’ll also want to take a look at this question for a rundown on dual wield mechanics:
(Your class also comes in to play here. For example, Demon Hunters using a 2h such as a Bow instead of a Hand Crossbow can still equip a Quiver in the OH, retaining the stat bonuses from the offhand slot).