I agre with cardinal sinner, a background and some java moss.
Maybe a couple of small plants at the front of the cave to break it up a bit.
On the whole though, I like it.
Nice tank! Like the rockwork. I think if you are looking for more depth you need more smaller leafed plants. This will make it look like the scale has been reduced.
I like it alot. If you want to change it... maybe try replacing a couple of the rocks on the right with a nice piece of driftwood or try a different type/texture of rocks like big rounded pebbles.
If it were my tank I would
1. change the sword in the front left with the one right next to your filter i.e. smaller in the front next to the rocks, and the bigger one would hide the filter better
2. Moss on the cave would be a great addtion
3. maybe lose the rock thats all alone next to the cave and replave with soth driftwood
4. move the thermastat to the back corner
Yeah i would move the sword to the right nearer the back, lose the rock at the front near the middle, grow moss on the cave and grow something like a few bunches of crypts in the foreground, promising start though.
I like your tank, give it time to clear and grow a bit, maybe add some smaller bushier plants around the base of the rocks, as somone had mentioned earlier, java moss would look great on those slate stones.
I think your tank has got great potential too. May I'd change the cave bit. Being so angular, it looks a bit man-made. Perhaps moss would help, I agree, also, more plants would make things a bit more wild. As everyone else has been saying, what's going in it?