It's a combination of factors:
1. The dimensions of the driftwood are such that the long branchy bit extends only about as far as the driftwood piece that's set in the substrate, so it adds a bit more symmetry to the tank layout than I'd like.
2. It's large enough to largely obscure the bit of blyxa that's planted behind it, but not full enough to really give an impression of filling in that back corner. It feels too 'open' behind it, between the new driftwood and the edge of the in-the-substrate-driftwood in particular.
3. Overall, it just doesn't quite "fit" with the rest of the layout. If I had been designing around the new wood from the get-go, I'd not have used the piece of driftwood in the substrate at all, and would have put the new wood about 4" to the right of where it now is, and an inch or two more forward. Maybe kept the blyxa right in ring around it, and have the HC run right up to that. And I wouldn't have used the hairgrass at all, most likely.
I dunno, I'm just not thrilled with the way it turned out. Just stumping around trying to think of a better alternative.