Rather than just say "whoah that's awesome!" and leave it at that, we would do well to examine and figure out why it's successful.
There are a lot of good fundamental concepts being put to practice here. This is important!
The brown characters are very easy to read against the blue backgrounds, since they are opposite colors. (This rule is broken in the interiors where there is less action so readability is not as important) There is a priority in detail - if you scrutinize the backgrounds you'll see that most of the small detail is actually more or less implied with pretty thick strokes since it is viewed as a whole, but the characters which will be seen constantly throughout the game are drawn with painstaking detail. This kind of visual priority saves unnecessary work but more importantly it keeps your eye from getting "caught" on things that aren't important without hurting the overall aesthetics. Solid understanding of light and volume illustration. Drastically diverse sillohuettes make characters instantly identifiable.
Snake's previous projects haven't been ugly by any stretch but this is the most cohesive work I've seen from him so far. Great example of how much core graphic design concepts can elevate an already skilled illustrator's work.