Before you read this post, read this one by Larisa. It's a good start to this topic.
There is a balance between WoW as a game and WoW as a world. They can coexist, but there are diminishing returns, so going too far in one direction will have a major negative impact in the other. WoW has been steadily becoming more of a game as time has gone on. It is an excellent game. This is why it is so popular: because it is a good game.
Unfortunately, this has come at the cost of being a world. Immersion has been weakened. How? On what do I base these claims?
Let's start with Larisa's desire for long instances. Raids also come up, both in the post and in comments, I'll leave those for later. What makes a long instance better? Well, immersion. It takes time to get oneself mentally into the world. Not many people can walk into an instance and instantly be in it mentally. Sure, you might be ready to start pulling, but are you virtually living in it yet? Probably not. Ever notice how pinball tables usually have some sort of theme and artwork? Do you care? I doubt it. In the early minutes of an instance it's practically like pinball. With some time you might get into it.
Except something can get in the way: the game. If you have to pull carefully and fight carefully with full attention, you can't notice the instance. Easy trash is essential. Trivial trash. All it does is slow you down, keep you from reaching the boos, but without taking your attention too much, leaving your eyes free to wander and see the art which is a well-done instance. And as you take it all in, you are more able to be taken in by it, immerse yourself in it.
Instance design matters as well. It cannot be a hallway. Or at least it cannot be an obvious hallway. Sadly, this was a problem in many BC instances. They were very obviously hallways with practically no indication of being part of a larger world. You were clearly in a small area, isolated, with no impact on or from the larger world. That's hard to get into.
WotLK does this better. It still has pretty linear instances, but they are loopy and have large areas which add to the instance but aren't part of your travels. Gundrak is a great example, with parts sticking out which don't make you run further or get lost, but which add a sense of scale, that you're in a larger troll city. But even this is not as effective as simple making a big instance.
Enter BRD. I mean that both as fake stage direction and as an order: go to BRD. On your way in, look around at Blackrock Mountain. Now go down, down down down, into the heart of a mountain. Deep inside. BRD is practically a maze and very long. You would have to try to not sense that it is a city. It has a bank, a bar, peasants, forges, an armory, a highway which seems to lead to an even larger area which has its door shut. Notice how it looks so similar to the mountain. This is not an instance, it is a city, a part of a world.
Here's the problem: BRD can take hours to clear and you can easily get lost, even after being in there a few times. The quests go all over the place and there's no clear path. These make perfect sense in a world, after all, could you easily navigate New York by a narrow path with only a handful of turns? But they aren't so good for a game. They are frustrations. In contrast the BC instances were very good as games: straightforward and understandable, with no chance of getting lost (I'll admit people still managed).
This goes beyond instances to the entire game environment. However that will have to wait, since I'd guess the demand for really long instances is about the same as that for really long posts: low. If you get bored while waiting for the next posts on this subject, hop over to Stabbed Up and check out the recent posts about RP. Presumably if RP can emerge from gameplay, so can the immersion of a world, it just has to be properly done.