Well, since you asked...
1) too symmetrical. Symmetry makes it "feel" artificial. The symmetry is in several ways: a) almost everything is low b) almost everything touches c) almost everything is on top or under something else d) there is a land bridge across almost the entire bottom e) almost the entire bottom is covered f) all the items except the one on the far left shows a "purpose"
Less is more in decorating. Create a pile of rocks, but have spaces between them. Create a cave shelter, but make it look like the rocks fell into place and weren't planned. Try to remove a couple of the most unique rocks and replace them with some that match existing rocks. Make caves but don't make them all the same size and same height and same distance from the viewer. Put something really tall in the tank like a plant or a big pile of rocks, preferably not dead center. Place a couple rocks in the tank with no actual purpose, like right in front, far to the back, etc.
2) despite the point above, the pieces themselves are too different. I certainly like of them and think they should be used, but they are too eclectic. Natural environments are rarely such that everything in a span of 4 feet is a large unique item. A homogeneous selection with a only 2-3 (at most 4) favorite pieces, imo, is more pleasing and natural.
3) cover the background (wires, wall)
4) also related to the above, covering most of the bottom (like in the pic) tends to encourage dead spots and mulm/detritus buildup which is fine if you routinely remove them to clean, but otherwise not.