it gets DR'd.
it really depends on your slotting and your specific toon.
DR hits on a per power basis, so 10% of a 500 point attack is 50 points, and for certain playstyles and toons, that's worth it (my fire/elec dom runs assault), but generally, maneuvers for the lotg and kb io is standard practice.
Barrier is correct.
Example of how it works:
Attack is slotted with 90% (.9) damage from enhancements.
DR is applied to the enhancement value. (Base damage is not affected, only the enhancement)
Then the damage bonus is added; 10-19% (.10-.19)from assault, buildup (.8), +damage IOs, whatever...
Then DR is applied to the total damage bonus.
As barrier said, the value varies per power due to however the power is slotted. I'm not sure what the cur is for +damage so I can't say how much DR's effect is exactly. However, it isn't terrible like it is for some attributes. Some things like defense and to-hit are forced into soft-caps more or less. +Damage does not fall into this category, if it did, BU and Aim type powers would be pretty useless.
I was wondering how hard +Damage from the likes of Assault and set bonuses gets DR'ed in pvp. Some have said it gets slammed so hard its not even worth taking assault as a power, others have said its one of the least DR'ed attributes in pvp. Anyone have a definitive answer about this, and at what percentage. 15%, 30%, 50% etc, it would start getting pointless to try and add more damage?