Like others have said, looking at features that have independently evolved numerous times on Earth should give us a clue.
I would say that sight is very important. Even in the darkest environments on Earth, such as the deep sea, there are highly adapted eyes. Heck, many of the deep sea organisms produce their own light through chemical bioluminescence. This sight should focus on wavelengths that other organisms would either absorb or emit, so we are talking about infrared through UV more than likely. Wavelengths below infrared can pass through organisms while wavelengths above UV tend to destroy biomolecules that we consider important for survival so they shouldn't be in abundance to begin with. We also know that two eyes is better than one if depth perception is important, and it is also handy to have at least one eye on each side of the body to look out for predators. So much like paired appendages was mentioned above, paired eyes also seem important.
Once you have sensory adaptations you also need a concentration of neural cells to hand that input. Now we are talking about speed, being that impulses will more than likely be chemical in nature and not electrical. Even in the most advanced species on Earth (us, for example) nerve impulses still travel at about 30 m/s, which is pretty slow compared to 3E8 m/s for electrical currents. The closer the neural cells are to the input the faster the reaction to those inputs. Nature here on Earth has shown us how important this is. All but the simplest Earth animals have ganglia of neural cells located near their eyes, noses, ears, etc. Therefore, I would predict that the "advanced" species would have a head of some kind that housed most of the important senses as well as a collection of neural cells to handle those inputs. There is always a niche for a predator that can react quicker to the presence of prey, and there is always selection pressures on prey to outmaneuver the predators.
So I would tend to agree with some sort of symmetry for locomotion, specialized tissues, and a head of some kind to house the most important senses (especially sight) and the brain to run it. Of course, this is for what we would consider to be advanced species. As we all know, millions of species get along just fine without any of these adaptations.
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