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.