My first comment would be about the lightsource. It seems to be confused somewhat.

If you look at the darker sides, (top right slope and top corner slopes) This would suggest a lightsource from somwhere between bottom left and bottom.

If it was exactly bottom-left then the right corner slope would be the same as the top corner slope, if the lightsource was directly from the bottom, then the top-left slope would be the same as the top right-slope. So whilst it is neither, the lightsource suggested (from the shadowed edges) falls somewhere between the two.

If we now look at the lit edges, then it would suggest a lightsource somewhere between directly left, and bottom left and, again, not actually exactly either, but somewhere between.

Hmm.. so which is it? The lightsource indicated by 'lit' faces differs from that suggested by the 'shadowed' faces.

As for the angles of the slope, that is ok (and even verging on the 'good' side of 'ok') - if you are planning to have a very uneven hilly surface. However, if the slopes are only few with larger flat areas, then a slightly steeper slope might help emphasise the fact that there is a slope involved.

You will really need slightly more shades of green than you are currently using, with a wider range than you are currently using to correctly shade the ground with increased contrast (for the faces).

-Z-