Didn't EQ have a few pure DPS classes that were left to the wayside because they didn't offer anything more to a raid?
EQ's pure DPs classes did lots and lots and lots of damage (read: you weren't getting close if you weren't one). The EQ classes that started falling by the wayside were the ones that had their damage gimped for buffs / cc's / summons that proved largely pointless in upper end raiding outside specific fights. You ran 2 (maybe 3, depending) enchanters because you didnt want to have to use more. You gained more by using a knight to offtank trash (which was all you could mez, and bards could do that too, and bards were just stupidly amazing if you found good ones, while enchanters were pretty craptacular unless they were god) than by using an enchanter as a non-tank non-healer spot.
EQ is different though. It has (or had, I stopped paying attention about a year ago) focus among its classes, and its hybrids were actually hybrids.
For instance, In the same zone, in the same gear (swapping from sword and shield to 2hander to DA hammer and shield depending), I, as a paladin, would be spamming Greater Remove Curse on the first fight, healing and light curing on the 2nd, tanking the 3rd, tanking/dps/curing on the 4th in different phases, healing/curing the 5th and healing/dps'ing the 6th. To do that in WoW... you simply cannot, its impossible. (For one thing, it would require a 43/48 talent build with permanent fear ward)
As I see it, the biggest problem with WoW is that it fails in equal distribution. This is to say that if you took 1 member of each spec of each class, geared for that spec's purpose, performing that specs purpose in a raid, your raid would fail.
This is one of the comparisons people are drawing to EQ, where in an equal distribution of 15 classes into a 54man raid left you with 3 primary healers, 9 secondary healers, 3 primary tanks, 6 secondary tanks, 6 cc'ers, ect ect ect. Enough to do anything.
WoW breaks 9 classes into 27 focused trees into... 140? sub-builds with a different focus. Surely there is a difference between a warlocks 41/20 and 41/0/20 and 40/21 and 40/0/21 builds. Thats just affliction base. But sticking at 27 broken into 25 raid spots (assuming the subtelty rogue and survival hunter are nowhere to be found), you're still short necessary classes (your healing is light, you have 1 shadow priest, and you're using some pretty bad specs all at the same time to boot. Its simply not feasible to use every class as is intended for them to be used (which is why you have a lot of grumbling paladin... still. And so on for most classes)
The underlying problem, though, is rather complex. To paraphrase an old professor of mine, "Reform just pretties up the problems". Why the Wow system of class balance is broken, and how that brokeness is core to the game as a whole, would be an adequate (if grammatically incorrect) title.
1. Talent trees disallow coherent customization for specific roles.
Talent trees are not designed as an AA system, and are instead designed to give players hoices in character development. However, the construction of talent trees result in taking several, or, in some cases, many, purely worthless talents for what you want to spec to. Additionally, for what are considered hybrid roles, some classes cannot properly spec to fill them adequately (the true paladin offtank healer requires more points than possible even if you just limit it to the snap aggro talent and the shield block talent sorely needed in this game due to mechanics, and ignore deeper talents, which make your hybrid more of a waste of space).
The inability to correctly talent your character COULD be made up through gear.
2. The inability to outgear the lack of talents is not allowable due to Blizzard's rather horrid itemization system.
Repeat after me. Mudflation is not always a bad thing. Mudflation allows classes the ability to function in non-pure roles. Most classes in Wow are hybrids. (you can stop repeating now). WoW's system does not allow gear to be designed for specific classes (its changing somewhat, with feral AP and sd based on +heal) but to truly build on this, you need a system that doesnt scale cost exponentially with stats, allowing gear to serve multiple purposes, and stay away from having to chose between 2-3 (and with druids, it should be 4) sets that are situationally useful, but limit hybrid classes to specific roles.
With the talent system in place to already check players, theres no reason a paladin piece couldnt have the same tank stats the paladin sets have now, with +healing and mana5 or spellcrit thrown on for 'no charge' (much like feral AP is/was free with spellpower) because you cannot, if you are getting use out of all of those stats, get maximum use out of ANY of those stats. See also shaman, druid, warrior, priest.
The ONLY benefit of modular equipment is the ability to change it in the heat of combat, to use the best for that particular situation, regardless of what that is.
3. You cannot change gear in combat.
This goes hand in hand with problems seen in #2. In fact, if you could change gear in combat, modular gear would be somewhat acceptable (though nowhere near ideal). You cannot, however, change gear in combat, nor will you likely ever (until the death penalty is changed up to something like taking a durability hit equal to 90% spread out across every piece of gear you have equipped during the last x amount of time before your death [or same combat as your death for scripted encounters].
Its also established you cannot change gear for another reason...
4. WoW is balanced around FAR too many different situations.
Quite honestly, and theres a billion examples floating around, you cannot balance a game around Solo and group and raiding and world pvp and 5man arena and large scale pvp and expect it to resemble what it would look like if balanced for any one of those separately. Even just saying PvE and PvP balancing, its still two completely different worlds (I could write a book on the specifics of balancing for each in WoW, but this post is long enough really),
Honestly, to properly balance the classes in terms of upper-end PvE, you really just need to gut the gear and talents of all but 2 concrete roles, and let the talents work on allowing completely viable roles of either of those specialized (ie. Paladin as healbot or tank. Shaman as a healer or a lightning mage) with a hybrid version of them (ie paladin as a viable offtank plus spot healer) as a third tree, with NO (none, nada, zilch) ability to take points from a different tree, with gear focusing on the hybrid roles (ie paladin stuff designed for a heal casting tank) to allow talent selection to be the deciding factor of uniqueness.
Its the attempt to not have a square peg (35+ unique subsets of classes) fitting into a smaller triangular hole (25 raid spots) with special notches (healer requirements greater than existing unique healer subsets)
This, however, is nowhere close to an elegant solution.
The other way requires an overhaul of balance mechanics (spells and abilities working differently in PvP and PvE), the deatpenalty, gear swapping, and several other core fundamentals of WoW.
They clearly stated several times at Blizzcon that the want there to be real distinctions between the classes and that it was for this reason that they don't have a lot more classes - then they introduced another class.
We almost got a second added, with another 2 more in late-stage development. Imagine the fun THAT would be right now.
edit2 - Math of balancing is another funny thing. 50% effectiveness tank + 50% effectiveness healer = spot on the bench. even 755 and 75% is quite often a liability, assuming that 25% is base and not added from unique talents.