I gotta throw in my 2 cents on this, cause it's something I'm very interested in!
One good principle is to avoid creating equal shapes (size, height, mass, etc.) And along with that goes avoiding splitting shapes visually in the center (either vertically and horizontally). And, avoid anything that grabs your attention that's dead in the center of the tank. It's more interesting to favor one side (of the tank) a bit.
Maybe a big mass of rock that's shorter and a narrower but taller rock mass. Some people like to use the seesaw principal; if you imagine the rock masses sitting on a seesaw, a bigger mass closer to center will balance a smaller mass that's farther from center.
Contrast come into it - horizontal vs. vertical, tall vs. short, big vs. small, color contrast, etc. On color contrast; if you put a purple acro next to a green one, both will look more vividly colorful because they're complimentary colors - across from each other on a color wheel. Another color scheme might be putting cooler colors at the back and warmer colors at the front.
Anything that "points", like a oblong rock, will pull your eye towards the direction it's pointing. You want the viewers eye to be continuously pulled, never coming to a stop. If your eye hits a rock wall, it will slide down it towards the bottom of the tank (or sometimes up if the rock is leaning differently), so (if your eyes go down) you want another rock mass (even a small spur will do it) or coral near the bottom that leads the eye back into another part of the tank again. Course, all this depends on what you want from your tank; some people want harmony and some others want a kind of colorful chaos? But, this "pointing" shape stuff is really helpful for achieving either one. If you notice what pulls your eyes around an aquascape, and start getting tuned in to that, you won't believe what a difference it will make.
Also, one really hard thing to do in most tanks is create depth, but you can do it with even a small spur coming forward from the main rock mass. Or, another really effective way to create depth is to put something narrow, like a gorgonian, behind an opening in the rock masses - anything to create layers of depth.
If you think of an aquascape as music; you probably don't want a monotonous beat, you want a little syncopation, some visual jazz. Some surprises.
As for avoiding even spacing - not easy to do since you want your corals to have room to grow and room so they don't touch other corals, but you can do it by just varying the distances between them a little, maybe putting a smaller acro a tad closer to a bigger one. Or slightly in front of another.
You don't want anything (rocks or corals) to be equal sizes or heights, and especially not a row of them.
But, you also want simplicity, and a way you can achieve that is by grouping. If you look at good aquascapes you'll almost always see things grouped into a few groups. If you look for that, I bet you'll see it right away.
Anyway, hope that's useful to you.
Not that I'm ever satisfied with my own aquascapes, of course!
