Try to think of a totally new resource system for your class. Maybe it's shamans using a mojo system* or paladins using something like DKs. Think it's likely? I doubt it. The problem isn't with Blizzard having no new ideas (DKs have a pretty new system) or with refusal to improve (they've radically changed a lot, but still within bounds). Ultimately the problem is with existing players.
How would you react if one day your class functioned differently? Imagine warriors using mana-type mechanics or mages with energy. Many people would be delighted. But I imagine many more would be angry at their class being changed so much without any reference points for conversion (such as now I think of str as the new SP for prot). The majority would probably just be confused. What happens to rotations? Do we use the same stats? What is this oddly colored bar?
A few times I've suggested converting holy paladins to using ret gear: strength and crit for the most part with haste, hit, and ArP as secondary stats. This is actually what triggered this post, my attempt to respond to Rohan's latest post about sheathe of light. In response to responses some ideas bounced around my head: hit increasing intellect, ArP increasing the effectiveness of healing, varying with armor: the idea being that paladin heals can somehow use not just your own gear to gain power, but also use the target's armor like a giant antennae, and somehow ArP would help with this.
Then it got me thinking, what if intellect wasn't an issue? What if paladins didn't use mana, but instead something else? Imagine a system like rogues with attacks using energy and generating combo points which are then spent on special abilities, in this case heals, like the Ace's High drakes. There would be a lot of trouble. People haven't just learned the mana system for paladins, it may be part of the mechanic of why they play and enjoy them. For example, my inability to manage energy is why I barely play rogues, so converting paladins to that system might make me reroll. Could an innovative new system, intended to bring new life to an old class, actually destroy it?
We've seen a lot of changes over the years, but I don't think any changed how a class works. Stat values vary, but ultimately we're using about the same stats. Even in the case of paladins, everyone is still using SP, they just don't directly stack it on gear. Bringing this back to my suggestions to move holy to ret gear, they seem plausible, but anything more, any true mechanics changes, would likely never happen.
The current classes are already defined and to change them would potentially be very unpopular. This means that we're going to see more ideas come from Blizzard, new ways to attack and heal and defend with new ways to manage resources, whether they are cooldowns, mana, energy, rage, runes, or something entirely new; but unless we reroll, we're not likely to directly benefit. If that sounds pessimistic, look at the bright side: any new class is going to be filled with amazingness, just as DKs are, except hopefully with better balance.
* The mojo system, or something of a similar trollish name, was suggested years ago on the shaman forums. The general idea was a spectrum of mojo: good and bad, with heals moving your mojo towards good and damage towards bad. If you get too far, you can no longer cast spells of that type, so you are forced to mix damage and healing. I thought it was a pretty cool hybrid idea. RIP hybrids.