Well, I thought of something else I didn't mention in the post on monks. Playing the class made me feel sorry for rogues, because the monk is yet another way in which Blizzard has encroached on territory that used to belong to the rogue.
The rogue is a leather-wearing melee class that uses an energy bar and combo points. When WoW was released, each of these characteristics was unique to the rogue (as well as the feral druid). But that has changed over time; first Blizzard gave hunters an energy bar, then they gave several other classes (paladins, Shadow priests) combo points.
Now the monk comes along; a melee class which wears leather and uses energy and combo points, but also has the ability to spec as a healer or tank. All the rogue really has left is the stealth mechanic, which is frankly useless outside of PvP. I don't believe it's a coincidence that rogues are currently one of the least popular classes in WoW, and have been for some time.
I don't have a solution to the problem of rogues. However, I do think this shows how difficult it is to add a new class to a game like WoW.
At its core, WoW is a game about reducing an enemy's hit points to zero before they can do the same to you. So the mechanics of a class in WoW are essentially an answer to a simple question: "How will this class deal or prevent damage?" And as the last decade of WoW development have shown, there are really not that many ways to answer this question that are also feasible, intelligible and fun to play. Blizzard has come up with some mechanics both old (rage) and new (stagger) that add new twists, but there is only so much they can do.
By contrast, it is relatively easy to think of thematically new WoW classes - bard, ninja, barbarian, enchantress, necromancer, knight, warlord, spellsword, psychic, vampire, sheriff, etc. Although there is some overlap in flavor between these and the existing classes, the overall thematic space is much larger than the space of possible mechanics. This is because the theme of a class is an answer to a much broader question, one which could be something like "What does a perfect example of this class look like?"
Since the space of themes is much larger than the space of mechanics, there are only two options: either not every theme can be represented, or some themes must share mechanics. With the monk Blizzard has chosen the second option, trusting that the theme is so strong that it's worth having it reuse existing mechanics. And it's possible that they are correct. However, it's clear that they can't do this with every class that is suggested.
I wonder if it would be a good idea to make an MMO where classes could specialize into trees which were thematically very different, but mechanically similar. In WoW terms, the rogue and the monk could be specializations of the same class, and share most mechanics like they do now. But thematically their abilities could be very different, with rogue abilities emphasizing cunning and brutality, and monk abilities emphasizing tranquility and balance.
This would allow the developers to draw on all the traditions connected to each of those archetypes, and perhaps appeal to players who are particularly interested in one of them. However, perhaps the idea is more trouble than it's worth, and it's better just to stay with a relatively small subset of all possible themes.