I would borrow from one of the great observationists of human nature, George Orwell and his Five Rules for Writing. 'Never use a long word where a short one will do.'

I feel this rule applies equally to both writing and design. It speaks of concise communication which, in the world of information overload we find ourselves, is more important than ever. Without clarity, a message can be lost or misinterpreted. Less is, truly, often more.

I find that working from this perspective helps produce products that convey the intended message without letting design, for its own sake, muddy the waters. Aesthetics are, of course, also very important. Communicating effectively requires a connection that things of simple beauty often provide.

Say it simply. Say it effectively. Make it beautiful.

On another note... I, also, fancy myself an observer of our species. I have found, over the years, that there are certain predictible and nearly-universal rules for how humans interpret what they see. Keeping these rules in the back of your mind can be a very powerful tool in effective communication.