Design Is What It Does
It's hard to take issue with the Jobsian position that design is how it works, detailed briefly in this post from design firm iA.
Most people make the mistake of thinking design is what it looks like. That’s not what we think design is. It’s not just what it looks like and feels like. Design is how it works.
There is truth in this, but how it works is itself dependent on more fundamental questions such as what it does and who it's for. One may in response quibble that this is the role of the product manager, not the designer, but these domains share considerable (albeit partial) overlap. In my experience, good design requires solid, thoughtfully considered purpose for the thing being designed.