This is a relatively new style which has been growing in popularity really within the last month or so, to the point where Manchester actually used it on their Christmas lights turn on posters around the city! Another real-world example of this style was used in Japan as a remembrance to the Fukushima Daiichi disaster (Pictured above).
The majority of the images above are from a blog called Geoaday where a bunch of guys are simply experimenting with form, lighting and how simple they can make three dimensional objects without losing their meaning. They are clearly very talented, its a misconception many people have that low-polygon modelling is easier than high-polygon work; attention to optimisation, texture space, form, shape, light, and what is left out can make or break a low-poly model.
I really like the combinations of colours within these pieces, they don't appear to always reflect a global ambient colour, but each has a different light setup so for example the trees dark shades can be a complete different hue of green to the lighter parts, but it works really well.
These two main images show the same scene in two different global illuminated light setups, something I can replicate dynamically in Unity through scripting and adjusting the ambient light setup within the render properties. A dynamic light system could breath a lot more life in to the game and is something I will have to look in to (as it wasn't in the original project outline).
I can look at how simple their character models are and try and apply this method to the cat character I will be developing, it looks so simple yet from a few tests I've done its incredible hard to replicate. Not only have I got to get the anatomy to be believable, but then the joints will have to be animated and need to be constructed in a way that it won't clip in to other parts of the mesh.
I really like how the water is rendered in the bottom right image, it gives real depth to the image and also makes it all more magical in how it shows its edge. I think I prefer this to the top left image, it seems to give off the qualities of water a lot better, and gives me the opportunity to detail under the water with animated fish/seaweed (and will make more sense for the player when fishing).