Without going on at length about it, I think this screenshot comparison speaks volumes on its own. It's sort of a UI evolution.

(click for the full image that demonstrates it best)

1999 = Asheron's Call
2002 = Asheron's Call 2
2004 = World of Warcraft
2007 = Lord of the Rings Online.

Note: Kudos to Jim @ MMOG Chronicles for compiling this image.