I like caves and crevices, expanses of sand and rock you have to look behind to see whats there. I also like rocks going up to the surface of the water, and never really understood the 6 - 8 inches of open space some people have at the top of their tanks.
Here is my current set up, when it was first set up:
The cave on the bottom right is actually a tunnel that you can look through from the right side of the tank, and the middle is actually more open than it looks. You can see all the way to the back of the tank along the side of the right rock structure.
The left side of the tank was concieved as more of the rock wall section of the reef, which is why it goes all the way to the wall of the tank. If I was doing it again, I would have left space between the rock and the glass.
All in all, I like this set up very much. It is open so it allows lots of swimming room, complex enough that you see more the more you bother to look, and has enough surface area to host a bunch of stuff.
Here it is a few weeks ago: