Great design is simple in theory, but complex in execution.
To elaborate, all that is required for great design is to approach the problem in a manner that allows the message to be read as quickly and efficiently as possible. In other words, the message must be read simply.
The problem is, this execution usually requires a more complex idea behind it. A simple solution is not necessarily an easy solution.
The problem with design is that it’s not a fine art. There are rules to follow. In the end it’s not just the client’s opinion of how it looks that matters or that makes it great. It’s how the work brings in an audience, how that audience interacts with it, how the audience reads it, how the information flows, and how clearly the message is displayed that matters. If those things fail, then the design fails.