I guess that every product manager knows that good design goes beyond aesthetics. Although for software products, there are less technical restrictions to the design of the user interface than to tangible products, good design rules apply to software user interface as well.
Dieter Rams is a well-known German designer. His “10 Principles for Good Design” are a very good read on what purposes design has apart from making something look good. I find the principles about being unobtrusive, yet guiding the user, highly important for any product. Vitsoe has nice examples for each of the principles.