In general, leveling systems in an mmo are bad. Content should be trivialized by pull instead of push, meaning that you are inclined to go to a new, better zone rather than forced to leave.
One of SoD's defining qualities is that at level 65, the game ceases to be level based. As a result, all new content actually lengthens the game, whereas in a level-based mmo old content is made obsolete. Levels in a commercial mmo force new content to be significantly easier than the old, hardest content in order for casual players to continue to advance. A non-level based advancement system still allows casual players to advance while they would still benefit from raiding, should they ever choose to. The last advantage that I can think of in this system is that a casual player does not need to be at the end of advancement in order to start raiding or getting better gear, whereas in a game like WoW you have to be level 80.
Your first statement on pull vs. push is exactly correct.
Thoughts on second statement:
While it is true that at level 65 SoD ceases to be level based, what is the difference in a level and "required number of AAs/tomes/relic spells" as a prerequisite to do certain content?
In a commerical MMO, the level cap is raised and/or more difficult raiding zones are added in order to do exactly as you describe. (push vs. pull) This does indeed have the effect of making the old "hardest" content obsolete for the playerbase at that step in xp'ing/raiding. However, this content does not become obsolete for the casual player, if he can join/be lucky enough to join a guild/party that raids that content. (Unless, in an example from Live, the content is removed and changed to higher level content for higher level players.)
In SoD, new content also lengthens the game. This isn't done via adding a new "level requirement," but other requirements are added in order for people to participate in most cases. (This where the AA minimums, tomes, and relic spells are taken into account.)
It is NOT the fault of either of these models, however, that the casual player must always be playing "catch-up" to be able to do what others have already done. Game developers must balance content to the abilities of the majority that will be participating in the content. Further, more time played by a player will equate to more time availabe to advance. (Gamer with little time...casual... to the gamer with more time.... non-casual.)
Many encounters in SoD cannot be done by level 65s without X gear/AAs/tomes that gives them X hp, mana, resists and dmg output, including the use of X spells (archaic/relic).
This is NOT directed towards you as an individual, but your statement from "Levels in a commerical mmo....in a game like WoW you have to be level 80" is not completely true.
In Live, you didn't have to be max level to raid the newest content. (If you did, then raiding would be impossible until after more level grinding PRIOR to going and raiding the new expansion's content.) Many guilds would also carry "pre max level" recruits on their raids during probationary periods. This wouldn't be possible if this was true.
In DAoC, you didn't have to be level 50 before you began raiding dragons if a spot was open.
Other examples exist, but I hope you see the point.
Further, new content is not necessarily easier to allow new players to advance. (I remember GoD content being released in Live. Some PoP raiding guilds had difficulty raiding some of the "easy" content in that expansion.) Every MMO has other examples of this.
Lastly, on the specific topic of casual player advancement without raiding. (One of the topics of this post.)
Many mobs in SoD may indeed be PH type mobs. Further, by vistachiri's statement, many "high tier equipment players" may leave "droppable" loot mobs in place on the server due to "the reward not being worth the effort."
But, the casual player, by the definition of being casual, usually has less than 20 hours a week to play a game. Further, those hours are spread across many play sessions.
Once a casual player in SoD hits level 65 (a task in itself with many groups not wanting Dalaya's Beginners/non-guilded characters in their groups), they face the obstacle of "what now?"
1. I will never get Relic spells.
2. I will probably never get Archaic spells.
3. I will probably not be able to complete many of the level 65 quests, because of setup time of a PUG every play session to go to zones that aren't "the best" for money/xp to receive many "quest drop items" that will always have to be /randomed.
4. I can continue to xp in "the best" zones to buy equipment from listsold/auction because groups are easily found for this.
5. I can continue to xp in "the best" zones to get AAs/tomes to improve myself. (But, I cannot get the archaic/relic spells this way.)
In conclusion, the OP's suggestion of all "boss loot" being made "no drop" would help solve one problem, but create many others in its place. (This is just like other issues I have written long posts about before.)
And, I'll repeat, I agree with Manguadi that players should be motivated based upon "pull instead of push." However, the reasoning, in my opinion, is flawed based on real data. Further, no matter how much balancing is done to help the person with less (time, high tiered friends, a high tiered raiding guild, etc., the casual player will always be left at a significant disadvantage...NEVER being able to acquire certain things. Thus is game life, just like real life.
However, if those unacquired things are required for said person to experience a good part of the game, then something is wrong with the balance. (Hence, the SoD playerbase shouldn't consider levels 1-64 as a tutorial, shouldn't "demand" <and, I know a few people already that have quit SoD over this...groups demanding the best haste, best focus, raego, etc.> the best spells be cast on them in place of commonly available ones, etc.) But, again, that is the nature of the game and life.