Let's distill current WoW dungeon-running down to its essenence.
The activity consists of pressing buttons in response to more or less arbitrary visual stimuli in which failure may occur despite the correct button-pressing response. Success will cause the incremental increase in the Score, which is a number. Increasing the Score will reduce the random failure rate, but cannot eliminate it entirely. One session of the activity will last between one and three hours, based on the frequency of failure.
Only an idiot would ever play that game. That would possibly be the worst game ever. Incidentally I may have described many arcade games, but let's pretend I didn't because I didn't mean to. So let's just say that beside some quarter-gobbling piece of 80s machinery, no game with this model would ever succeed. Not even WoW.
Skinner boxes will give a reward we like. If we don't care about the reward, it won't work. This still applies in WoW. The rewards we get are not inherently desirable. Let's try this: would you be excited if I gave you a free Titan in EVE? Probably not. Odds are you don't play EVE. You may not even know what a Titan is. It is meaningless. It would not be a reward for you. What if I offered you a full set of whatever tier we're at? That might be of more interest. Why? Because odds are, if you read this blog, you play WoW, so that reward has meaning.
It is all just pixels. Meaningless and worthless except for the meaning and worth that we give to it. This is more than just answering the question, "what is a Titan?" You can probably tell from context that it is something really awesome in EVE. And yet you still don't care about getting a free one. That's because EVE itself carries little meaning to you. Maybe you never played it, maybe you played it and didn't find it to be much fun, maybe you do play it and cannot actually use a Titan yet. In contrast, WoW is a world in which you are probably immersed. You know stories, both personal and lore, and so the world itself has meaning. From this meaning the items in it, the attempted rewards in it, gain meaning.
What this boils down to is this: virtual rewards cannot have any more value than the virtual worlds which give them. This does not mean the rewards must all gain that value the same way. One person might value the appearance. Another might value the memory of how they got them. For a third person they are bragging rights. Note that the second and third person require some aspect beyond the reward itself. The second person needs an experience, perhaps social. The third person needs a challenge, perceived or real (virtually).
In other words, rewards in WoW (or any other game) are weak rewards, but powerful symbols. On their own they are pixels, but they act as symbols of how you got those pixels. As analogy, a medal of honor is what? A few bits of metal and cloth. But it symbolizes something (about the earner) far beyond those physical characteristics.
This raises the important question: What are rewards in WoW symbolizing? If they symbolize challenge overcome, social experiences, or perseverance, then they may retain their value as symbolic rewards. But if they are symbolic of a long time spent failing with strangers without quite being challenged or entertained, that might not be a very effective symbolic reward.