This whole argument about whether Defenders need more damage being broken down into semantics is mind boggling to say the least.
There is no doubt that the amount of damage defenders do, especially at lvl 50, is painful to deal with to say the least. That said, it is very hard to determine how much damage is enough.
Should an empathy defender be able to 1 shot an even con minion with a basic tier 2 attack? Probably not. (I chose empathy for the lack of self damage buff and lack of enemy debuff).
Should they be able to solo at the same clip as a blaster? I honestly cannot decide, but lean towards no. The reason being that if the kill potential is as great for both ATs, why ever team with a blaster? I can instead pick up a defender for de/buffs and damage.
Also as has been brought up, soloing is a very different affair depending on primary. Soloing on rads, kins, darks, TAs, and even storms isn't undoable. However, emps, FFs and sonics all have nightmarish times. This is almost entirely based on whether the primary power pool is buffs or debuffs. What always interested me about the concept of being able to solo as a defender, the two "strongest" blaster primaries are not available to defenders. I often wished I could go around causing havoc with a Kin/Fire or Rad/Ice defender, but these options aren't even available. This makes me think that it was always designed to keep defenders from becoming too offensively potent.
As how to fix this, it has to be a vigilance chance. Frankly if it added both endurance and damage in a manner of some team defiance, that would be one option. But then again, it doesn't reward a well built team where no one is at risk of dying. I would prefer a sort of power build up defiance, where every attack gives a slight boost to secondary effects other than damage. Or, as has been mentioned, the opposite. Buff/debuff/heal powers cause a boost to damage, as if you were feeding off the powers you were giving to others.
I think some sort of change like that would greatly help the oft neglected secondary power pools of defenders. Yet, I do resist complaining too much, as I rarely have issues finding teams that wouldn't love to have a defender along.