Unlike Nils I would argue the point not feature by feature, but rather by looking at the Bartle types. If you were to take the four Bartle types Achiever, Killer, Explorer, and Socializer and you would attribute two of these types each to "hardcore" and "casual" players, I think most people would agree that casual players are more likely to be interested in exploration and socialization. And World of Warcraft over the years has become a less social world in which there is less to explore. People don't get to explore leveling zones any more, because when they enter the zone and swat a fly, they get 1 million xp and a message that they just outleveled this zone. The high-level zones of Cataclysm are admittedly slower, but their extremely linear structure makes exploration unfeasible as well. If you were to try to explore a Cataclysm zone, you would either find empty areas which will be populated by NPCs only once you are at the correct point in the quest series, or you would find NPCs with whom you can't interact unless you reach that point in questing. And while I am not quite as negative about the Dungeon Finder as Nils is, I do agree that it comes to the detriment of socializing and making new friends.
Now obviously part of any MMORPG getting less interesting to explorers over the years is that content to explore is limited. "Been there, done that" is the death of exploration. But to reinterpret what Nils says, many of the features introduced to World of Warcraft over the years are more likely to appeal to Killers and Achievers, than to Explorers and Socializers.