My first impression is that this image is “washed out” Due to the lack of contrast I can’t see what you have created.
Sorry, but not much…
There’s so much overall ambiant light that I can barely make out the main Sun shadows. Are you using AO, a light rig or a Gamma node on this sceen?
My wife saw this and commented that it was “murky” - which is probably a better discription then I used.
It is supposed to look like it’s deep in a forest… the murky look is because I mixed a blurred version of the render with the original in the node editor. I am using AO.
Maybe try applying “murkiness” according to the Z value? It looks like an accidentally exposed photograph because the blurriness is relatively constant across the frame. If you deepen murkiness by depth, maybe you can create a foggy look to it.
I don’t mind the low contrast, btw. Murky scenes should be that way!
For some reason, the z output of the render layers node is faulty!!! I can’t get it to do anything; it is either all white or all black, no matter what I do with the map value node (with a viewer out its butt). I can’t mask the blur with z, then. Unless someone knows the problem?
That doesn’t look like a forest at all. It kinda looks like the underground woods in ‘a troll in central park’. You need some trees, also instead of making blanket grass of one colour, make the grass in tufts, then you can blanket in the middle where it looks bare. Also, what you have done there is put a sandy colour on the ground planes and then put green grass coming out of it. This is a noob mistake, but one we all make, don’t worry. Go outside and have a look at the grass, you can’t see any ground. This is because the grass blades all fall over and grow around each other and crisscross so much that you can’t even see the ground at all. It’s better off to make a few particle layers to simulate this, or (the easy way and my favourite) get some high res grass textures. They don’t even have to tile, you can take photos yourself, it’s only to obscure the view and add to the suspension of disbelief.
As for the overall composition, I like where you’re going, you have a focal point (the cave) and a hint of image framing with the grass on our side of the lake and the rocks and long grass above the cave to either side. You need to make the cave more interesting though, so that once the attention has been drawn there, it has a reason to linger. Otherwise the image will be bland. For one, get rid of the grass going up the walls. It doesn’t look convincint that the same grass is all over the cave too. I think a good way to keep attention at the cave would be to make the opening a bit bigger, and get rid of that lighting in there. The cave has no contrast with the rest of the image. I think putting some glowing eyes or something in the darkness would be interesting, although you might want to avoid the cliche.
I have no problem with the single directional light, but I think since it is a dark forest (assumedly) and there is mist everywhere (apparently) then the light should not only strike stark contrasting shadows, but it should create light rays (or ‘god rays’). These can be modelled in and UV mapped with a semitransparent texture easily, and I think in this image they would give quite alot to the complexity of it.
One last thing, although you have a bit of fore ground going on, I think that the picture would become even more interesting if you added in a tree in the very foreground with some vines hanging off or something, so you can only see a portion of it’s trunk and the tip of a branch with the vines. As if the photographer was standing a meter or so back from the tree, or even close enough to be leaning on it.
I like landscape renders, and with enough effort, they can be made to look really good.
Keep blending!
Thanks for the great advice! I have some light rays already, using spotlamp shadowbuffers. But I guess fake ones are fine for a still image…
The first thing that struck me is that i can’t see much at all on the picture 
I’d say render a double sized one and add some contrast !
Dunno if you post-pro’d it already, but anyway.
