The main issue I've found here is that I'd need to add more clear definition on corners.
On a 3D object you get a lot of visual information from perspective and form of the shape. You can clearly see where corners are, and when objects are separate from one another. In 2D you need to add edge lines and differences in colour to get that same level of differentiation.
I'm also finding that there's not such a clear difference between floors and walls as they're sometimes similar values. Here's a good example of what I'm talking about:
The wall in the centre of the image is not clearly distinguished against the floors around it, mainly because they share a lot of the same colours. This isn't a problem in 3D because you can clearly see that the floors are horizontal and the walls are vertical.
That's not to say this is an unsolvable problem, but it would require redesigning the tiles quite a lot and that would take time. 😉