My views on this have shifted significantly over the years but I would like a truly in-depth discussion on the matter. Damage dealing tends to be a key aspect in MMO's, but in AO more and more professions are reaching points where they have the option of doing exceptional damage. What we consider damage dealers tend to do 30% to 50% more damage than other players in most situations. This is honestly a low amount and does not allow damage dealers to form a niche to which they become necessary for that aspect in pvm. It seems to me what we then need is to give the role of damage dealer to a set group of professions, applicable primarily in pvm damage.
First is deciding how much damage the damage dealers should be doing, and what the trade-off should be. Also, the average damage ranges for everyone else should be assessed as well.
I feel 120k to 180k for tanks, debuffers, supporters and healers would be appropriate at 220. The damage dealers should be in the range of 350k to 450k DPM, so significant in the amount of damage they deal that their contribution is desired. That damage could be boosted even further with the help of support professions, making damage dealers really stand out in an encounter.
The trade-off that makes the most sense to me is greatly diminished survival. This means that a tank is not only necessary to hold the damage, but someone with taunts strong enough to hold aggro is necessary to prevent aggro from switching. AOE damages or mob damage shields would create a need for support healing and encounters could definately have debuffs which greatly weaken a damage dealer unless proper support professions are their to remove these debuffs.
There are many more improvements and aspects that can be added by making damage dealers a key niche with such a difference in possible damage.
Second is deciding which professions should take on the role of damage dealer and whether other roles should be possible.
The first obvious choice would be shades. While their damage is already exceptional it is often only through extreme measures and sacrifices that a shade can do noticeably high amounts of damage. Boosting the damage further at the expense of any supporting abilities such as init-debuffs or AAO/AAD drains seems appropriate. Full out damage focus while disregarding things that are mostly beneficial when the shade is alone. This is an example which I would expect all damage dealers to follow.
Other professions I would reccomend would be NT, Engineer, MA, soldier, agent and advy. Agents and adventurers are more-or-less going to have the option of filling any role, so the main focus should be the other four. These professions should not be tanking or filling any primary support or debuffing roles if they are to be doing exceptional amounts of damage dealing, it just eliminates the need for more players in an encounter. Soldiers may have the biggest issue with my opinion on this matter, but if they are already needed to tank then they should have the capability to hold aggro through taunting and not require excessive amounts of damage dealing. If the soldiers wish to do damage however, they must simply give up the role of tank to another player. This seems fair does it not?
Third is dealing with the issue of PVM content itself and the standard method of XP/SK/AXP grinding.
The primary source of AO leveling is through grinding content, mostly involving killing a lot of random mobs as quickly as possible. This means that the more damage you do the faster you level. While I do not mind this as an option, I think it hampers a lot of beneficial changes when this becomes the only viable way of leveling. The daily missions do offer all professions a means of steady leveling that does not focus on dealing the most damage possible, but it still falls short of standard DPM methods of leveling.
I believe leveling content should focus more on what I would call "epic leveling encounters" where they are more or less raids requiring teams just as any other raid, but the reward is large amounts of experience as opposed to loot. The reward would be larger than a daily or inferno mission reward. Having these as the prime means of leveling would help all professions if they worked under the same fundementals as a proper full team raid. Under those, for players that wish to level alone, should be questing and daily mission options. All questing should be possible for any profession, or at most require a single complementary profession to complete through the low to mid levels. There are ways to allow a tank, healer, and damage dealer to all do the same questline while having parts of their toolset make things doable in the same amount of time, therefore eliminating the reliance upon damage dealing.
For lower levels a simple grind-to-level style of pvm could still be highly viable, but the mobs should be low challenge and die quickly to any player. The heckler grind as an example is ok but we should really be moving on to other options after around level 120. The inferno missions, which are the primary source of leveling from 160 to 220 are simply a heckler grind inside an instance.
Fourth is the highly important issue of seperating PVM with PVP.
This is a very simple issue but I know it is a point people will raise. If someone is doing 450k DPM and then enters into a pvp situation, it does seem that they would be doing far too much damage to be manageable. In contrast, if we reduce player damages for non-damage dealers, we would have issues where these professions could not kill others in pvp if not handled appropriately. The solution is to simply make damage dealing toolsets which work only or at least differently against mobs than players. Detonation Matrix is a good example of a tool that accomplishes this.
These are my thoughts on the matter which I hope others can take time to read and remark on. I know it is hard for some people to imagine not only doing what seems to be abyssmal amounts of damage, but even losing further amounts of damage, but I simply ask that players consider whether they want pvm that requires everyone to do lots of damage to be bearable or pvm that is tailored to make use of every profession's toolsets to get things done.