After giving up publicly, I thought to do some actual research and here's what I came up with:

What the post-processing effect in GW does is this: It adds sort of an overlay on top of the normal rendered image which makes the edges of objects seem "faded" little by little into the background, instead of presenting hard (even if antialiased) edges -- look especially at the vegetation to observe this. It also adds a "glow" effect which is used to make certain objects (or parts of objects) like armor and weapons "gleam" and shine in the light or become more brightly highlighted. All the objects seem to have a slight "halo" around them and the image is much smoother.

This ads greatly to the visual quality of the game. :D

http://74.125.95.132/search?q=cache:y_2p59BS36cJ:guildwars.i...

Эффект постобработки, который делает яркие участки рисунка ещё более яркими, и добавляет свечение вокруг этих участков (ореол). Это кинематоргафический эффект, потому что он придает изображению схожесть с фильмом...

http://games.cnews.ru/eterm/index.php?filter=term&id=98