Two-handed fighting:
Bonus to speed (or glancing blows/off-hand attacks):
Adds glancing blows to attack sequence
+50% glancing blow damage
+9% glancing blow triggering weapon effect
Bonus to damage:
+50% stat modifier to damage
+20 melee power
Single-weapon fighting:
Bonus to speed (or glancing blows/off-hand attacks):
+30% combat style attack speed
Bonus to damage:
+10 melee power
+50% appropriate ability score to damage
Greatly synergizes with doublestrike
Can equip offhand orb
Two-weapon fighting:
Bonus to speed (or glancing blows/off-hand attacks):
80% off-hand attack
Bonus to damage:
NONE
Benefits high bonus damage dice such as Sneak Attack
Secondary effects in offhand apply to mainhand
***
Above was compiled with what others have written and my own sense of the merits.
I think the general player consensus is, yes, TWF has some uses for pure/near pure rogues, but beyond that, it's now mostly flavor for tempest rangers. I find that a little sad, but there was a time when TWF was king, as many remember.
My note to the devs would be...BUFF TWF this way:
give it a melee tiered power boost like other fighting styles
sprinkle enhancement trees and epic destinies with a few more TWF perks, on the lines of tempest or Balanced Attacks (Primal Avatar)
I'm sure there are DDO players out there with better ideas, though.