The Guild Wars 2 site has been updated with a new developer post that lays out some of the details about forthcoming changes to the world vs world system. Called "culling", the development feature made some enemy players invisible. With the upcoming patch, culling will be turned off completely.
Normally as you move through the world, any character (either an NPC or another player) within a fairly large circle around you gets reported to your client. This lets the client draw the character, allows you to select the character as an attack target, etc.
In order to cut down on the network resources that were used by the game and to reduce the client-side system requirements, we implemented a culling system, which imposed a limit on the number of characters that could ever be reported to your client. This meant that only the characters closest to you would actually get reported.
Read more about culling on the Guild Wars 2 site.