Very beautiful looking tileset scene, however the rock walls still seem to feel a bit 'flat' and painted on. Somehow the jaggedness of the surface feels like more of an optical illusion. I think this is not helped by some of the 'top surfaces' of the rocks not really being at a true isometric angle but seem to always slope outward from the tile surface. While I love the colours of the rock and grass, the rock highlight does strike me as a bit too orange.
Also.. why use square/rectangle tiles for a triangle/diamond grid? I'm pretty sure you will end up needing to make far more combinations of tiles merging one tile to the next. (
http://www.miniwizardstudios.com/iso-tiles.asp)
As for slopes, while the current inclination you have is neater mathematically it can pose certain visibility issues of lower tiles 'behind' higher tiles. You have a 1, 1, 1 slope there and the natural iso metric is of course 2, 2, 2 (number pixels across with each 1px raise). A 1, 1, 2 (although harder to hide the jaggedness) is half as steep and allows better terrain visibility (4, 4, 4 on the reverse slopes instead of a flat line)
----
Edit:Quick edit/example of using triangular based tiles for constructing the image with the 'iso-blocks'.

Note: the first two (split) blocks would have their side face covered - unless you only show a fixed area instead of bleeding offscreen. I haven't bothered to show a soil cut-through, but just blocked in very roughly to show the shape mostly.
Whether this method or your square tile method is better would be an interesting debate. The obvious advantage to your method being no 'wasted space' in a tile sheet.
-Z-