When I ask this question I'm referring to any and all aspects and elements of dungeons: their length, difficulty, scope, mechanics, artistic direction, questing, and so on. I'll mention some of my own for a start, and invite you to share your own opinions.
In my personal opinion the dungeon experience, as opposed to raids, is a significant component of the gaming experience. During Wrath, I loved that they added new five-man dungeons even as significant raids (ICC) were on the way. For me, I feel like longer dungeons, while exhausting, lead to more immersion, and immersion becomes in greater and greater demand as gaming convenience increases. I also feel like the "all quests available in one-go" model is conducive to convenience and therefore takes away from immersion. I really think we should go back to long chains that take you from one dungeon to the next, in and out, and require multiple dungeon visits. You might think that this makes the game more grindy, but in most cases while gearing up players have to do those dungeons multiple times anyway. If anything, a complex questing system adds to the storyline, adds to immersion, and increases replay value for those dungeons; as does making them longer, which might take a little more time to complete, but is far more rewarding. Also, considering grouping up is a lot more seamless today than it used to be, longer dungeons aren't such a bad thing anymore.
I also feel like a genuine focus on creating epic and immersive dungeons, especially ones that link meaningfully to the overall expansion plot and what's happening outside those dungeons, contributes positively to the casual-versus-hardcore raiding dilemma. As far as that conundrum goes, I do believe that certain content (and by content I mean actual content and not just difficulty) has to be earned; it increases immersion and enlarges the game world's perceived size in the players' imaginations, something no design-based-work-around could ever compensate for. With that being said, creating meaningful and challenging dungeon experiences provides players with a lesser skill level than master raiders a platform to explore the universe further, and a stepping stone towards becoming better players and consequently earning that skilled-raider-exclusive-content themselves.
I would also add that high-level zone content should build towards improving player skill in anticipation of harder-than-now dungeons, just as dungeons should provide players with opportunities to prepare for harder raids. In other words, bring back elite quests that at least invite players to work together again (which should work hand-in-hand with the realm-merging feature that's incoming). Remember the Nesingwary quests and the "Ring of <blah blah>" questlines? They made people come together, even if briefly, and rewarded them for it appropriately.
In conclusion, all I'm trying to say is that all game design elements should work together holistically to improve the gaming and storytelling experiences, and to collectively guide players into a process of gradual self-improvement, rather than function as separate elements of the game. Dungeons, in my opinion, should be longer, more challenging, and have better and more consequential lore, as they once did, and the rewards in terms of both loot and immersion will also improve. Seeing as indirect technical elements of instanced gameplay, such as group creation and member replacement, are no longer a hindrance, this should be a smoother change than many people expect it to be.
P.S. In anticipation of "nostalgia" attacks, I would comment that, while I cannot stand before my fellow gamers and deny nostalgia, its presence doesn't confound critique and commentary based on actual observation. Consider this analogy: if you were in jail, you would certainly miss the years you were free. Would nostalgia be involved? Yes, but does that mean there are no other, valid reasons for missing your freedom?
Thank you for reading, please feel free to share your thoughts. I'm in search of dialogue and not conflict.