Sorry to revive this dead post, but I just want to say that I was wrong.
I've started to use my 2h Greatsword pretty much all the time (outside of group) now. I have found it to be more efficient when soloing. Lower mobs were a given, but I started trying 2h against even con 3-dot mobs (which I always went s/s before), and still performed better with my 2hander than s/s on most occasions. Heck, my 2-hand slashing still wasn't even close to maxed yet. I found a pretty efficient way of doing it as well to maximize damage and minimize incoming damage, which helped.
Also my comment about the DPS, I was only talking about weapon DPS, not what it scales to on your character sheet.
So basically 2h > s/s. But with higher-end mobs and group situations, I'll still use s/s to make things a bit easier on my healer.
I must say, I'm kind of happy to see this.
DRK screams a class for 2h weapons IMO.