I like it.
Very nice first start.
The grass is actually pretty nice I think. It could maybe use one darker shade, just sprinkled towards the roots of your grass clumps. Otherwise, I think it's quite nice.
one thing you will want to do is make variations of that grass tile. Right now, when you combine your grass tiles, it looks very obvious that it's one tile being repeated over and over. The same shapes and artefacts are visible where the files come together, and this looks unpleasant. The best way to resolve it is to make variations of the same tile, and draw them in such a way that they connect interchangeably.
An easy way to accomplish this is covered here. http://www.wayofthepixel.net/pixelation/index.php?topic=9865.msg107103#msg107103
I realise your tiles are isometric, unlike those in vierbit's example, but the same concept applies.
As for your rock and foliage, they are nice starts. I think your flower will look more like a flower with a simple palette shift. The orange doesn't read very well against the yellows of the grass. Also, make it rise from the grass further. That will make it read more like a flower.
The rock looks nice enough right now, assuming your game is going with a more flat-shaded, cartoon-y feel, but I would recommend dithering your highlights on the rock, or perhaps adding new shades. Right now, it lacks shadow. It could actually look really nice with some purple shadows, as it would complement the yellow-highlight grass quite well, but that's a big stylistic choice you'd had to stick with in shadowing everything else.
Great job overall, and I'm quite eager to see your improvements.