I see this misconception a lot, and art is an important thing to a game. It's the aesthetic and it's one of the biggest parts of Guild Wars 2. The Aesthetic is very stylized, very easy on your cpu and very easy on the eyes. Realism isn't the goal, but rather beauty is. I see far too often the automatic assumption that Guild Wars 2 is a Korean MMO (Of course it isn't ArenaNet is settled in Seattle USA) and I'm making this topic here to compare the two art styles: Western Traditional Art, to Eastern Traditional Art.
Our first video here is a slide show of concept art from Guild Wars 2.
Here are some key things to note on the art style. Notice the colors and lack of defined lines. Any lines used are created by color and shape, not the literal existence of an inked line. On the subject of color, that is one of the main differences in the styles. Colors are the king in this Western style of art, they create the form, shapes, emphasis, balance, and rhythm of the pieces.
Guild Wars 2's planned aesthetic is based on it's concepts. The concept work you see, it what the game will ultimately look like (Minus the fact that concepts are traditional mediums or emulated traditional mediums and 3d models simply cannot embellish all that the concepts represent and convey to the brush stroke.) I believe this is why people assume a Korean/Eastern art style because rarely games are made with this kind of aesthetic.
Here is another western concept slideshow from Skyrim. Not an MMO, but I think we'll all agree that it's, in fact, a Western RPG.
Notice that lines aren't obvious once again. Colors and the way the pieces are painted create the lines for us. Skyrim is very different from Guild Wars 2 in it's aesthetic, but you'll notice that the concept pieces are still very close in style. Skyrim simply went for realism while Guild Wars 2 chose to follow their concepts closer to create the stylized Aesthetic.
Now for the Eastern Style. Eastern Traditional art. Here is a slideshow of concepts from Square Enix's leading Art man, Yoshitaka Amano.
You'll notice that lines are more prominent and tend to overshadow the lighter colors. Less color is used to create emphasis on the bolder colors. The lines sweep and create motion in the piece, leading your eyes.
Let's use another game that created an Aesthetic very close to it's concepts in the Eastern Style: Okami.
Lines dominate the art, they are king of the design while color takes back seat to help define and not to be at the forefront.
Now here is an example that blurs the lines. Aion. While not a great game, it holds elements in it's concepts that resemble both Eastern and Western Styles.
I'll direct your attention towards color once again. Colors are light and are used to create emphasis on parts of the pieces that are darker. However, they lines are created by the colors as well, giving it a western aesthetic while still being Eastern in spirit. Aion was very much trying to be a Westernized RPG with it's concepts, which may be why Guild Wars 2 Aesthetics have people fooled about it's art style (Being overseen and distributed by NCsoft doesn't help.)
Thank you for your time, I actually enjoyed making this thread as it was a good art study. Hopefully you can see the difference in Eastern and Western art styles now, but in all honesty we're in a world of blending cultures and it's all but natural for some art styles to meld, Aion is a good example of this. Just don't assume on a glance whether a game is from the states or overseas; to be honest, it doesn't really matter.
NOTE - This was a comparison of Traditional PAINTED art styles, not cartoons and mangas. If you want me to point out the differences in those, just ask, and I will, but I'm sure we can all tell the difference in Aesthetics when it comes to comparing shows like G.I. Joe and Death Note.
EDIT - I realized I used a lot of art terms that some might not be familiar with. Here is a link to the Elements and Principles of art. http://www.projectarticulate.org/principles.php
TL;DR Western paint Styles have a stronger use of color and tend to avoid drawn, obvious lines. Lines are formed by shapes and form. Eastern paint Styles emphasize lines and use shapes and colors to compliment but not be the focus. These styles can be blurred and mostly depends on the artist himself/herself.