This is an aerial battle scene from Perfect World. It depicts how the z-axis is displayed while the user is in midair. Notice that the ground and sky are both not visible, even though creatures above and below can attack from those areas without the player noticing.
This image depicts where the z-axis starts to cause problems in Perfect World. This character is so far above the character below him, that character has become completely invisible. However, that doesn't stop him from casting spells on her. Also, while it may not be noticable in this image, the line of sight is somewhat poor when facing the ground - despite the height, the player cannot see very far in any direction.
This image depicts z-axis problems with navigation in perfect world. A player is following this character, though the following character is at a somewhat higher altitude - because Perfect World's z-axis is monstrously large, a player could be so far above you that you could not see them, however, still pinpoint their x-y location. Markers, maps, etc. only consider the x and y axes, making it nearly impossible to locate anything not at ground level.
Additionally, notice that in Guild Wars, a small pebble could distrupt your path, but in perfect world, the terrain is far smoother. You can literally fly at ground level and not be concerned about blockages.
Perfect World Visualizations
This is an aerial battle scene from Perfect World. It depicts how the z-axis is displayed while the user is in midair. Notice that the ground and sky are both not visible, even though creatures above and below can attack from those areas without the player noticing.
Additionally, notice that in Guild Wars, a small pebble could distrupt your path, but in perfect world, the terrain is far smoother. You can literally fly at ground level and not be concerned about blockages.