As others have pointed out, the rockwork looks a little artificial. The biggest thing that stands out for me though is the wood - can you angle it at all so it's at more of a diagonal slope? You could stack up some of the more slab-like rocks underneath it, or as Byron suggests - have it in a corner rather than bang centre.

My best advice is to look at some professional aquascapes to get some ideas as to what you want to achieve with yours...assuming you're wanting plants as well? This is a basic starter guide but you get the idea re: composition.

Here's a couple of scapes with a large piece of wood: