Rock Paper Shotgun has an interesting piece up looking at travel in massively multiplayer games, and while the analysis is really about travel in all MMOs, of course World of Warcraft gets placed front and center -- with the notable exceptions of Mages and Warlock summons, it's a game that squarely places you in its vast world, and asks you to make some solid decisions about where you want to be. While travel has certainly gotten easier (and will continue to do so), it's still an important part of the world -- sometimes, when you're in a backwater zone and your hearthstone is down and there's no summons available to you, you've just got to get on a griffon and put the time in to fly around.
Why is that? Why can't we just teleport around at will to places we've been before (a la Fallout 3 or Fable 2, if you've ever played those games)? Why does Blizzard make us traverse the wide world? RPS lands on two solutions: either they just want you to play the game more (certainly possible, especially since big worlds with long travel times and subscription fees are a trademark of the MMO genre), or they're just being jerks about it. But their panelists, and Blizzard, have offered one more suggestion: they want this world to feel vast, and one way to do that is to make you move around it rather than warp anywhere you want at a moment's notice.
Then again, that's some deep psychology, and sometimes you just want to get in an instance with your friends and fight (hence the recent changes to summoning anywhere, queueing from anywhere, and so on). Travel definitely serves a purpose in MMOs, but the genre has shown in the past few years that while instant travel all the time might shrink the world a little too much, sometimes you just need to get to where you want to be.