I think the main reason retail World of Warcraft fails to replicate the success of vanilla is because it's too hardcore. Or rather, because it's developers are too hardcore.
The current game director of World of Warcraft, Ion Hazzikostas, used to be the GM of Elitist Jerks. In other words, he was the guild master of what used to one of the best guilds in the game, one that successfully cleared Naxxramas 40-man back in the day.
Temporarily going off the topic of Ion, the major difference between classic and current Warcraft is this: classic was simple but time-consuming, retail is mostly simple and quick with some very hard components (raiding).
In modern Warcraft once players reach something they can't progress past, in this case heroic/mythic raiding, they quit. They either aren't good enough or don't feel like they're making as much progress as they would like. In vanilla Warcraft players could continue as far as they liked. The gameplay was simple enough for anyone to go through, but the content was drawn out enough that people wouldn't run out of it.
The conclusion you can take from these two scenarios is important. If a game makes a player feel inadequately skilled, they will be offended and quit. If it makes them feel like they need to spend more time in order to be the best, they will spend more time. Our ego protects us from the truth of a lack of mastery/skill in raiding by reacting like this, despite how both options involve time investment.
But can you really say this thought process is true for Ion himself? As stated above, he was one of the most hardcore players back in the day, and continues to aid in the development of incredibly hard mythic content today. Such a person would be likely to consider challenging yourself and defeating the most difficult of encounters a great accomplishment. Consequentially, they would likely find it internally satisfying to do so. This is the problem with retail's design.
If the game director derives accomplishment from skillful play/difficult content, then will he understand how players who don't do so feel? Players who derive accomplishment near exclusively from external factors are foreign to him.
tl;dr - game director derives accomplishment from more internal sources, and as such doesn't understand classic, which is diametrically opposed to that idea of how to give the player accomplishment when in comparison to current wow.