I think usability is so hard to get because there always seems to be a better way to do it. I could come up with an incredibly innovative and understandable method of communicating my ideas or allowing the user to interact with the interface, but there are simple things that may get in the way such as borders. It's hard to focus on every little rule we've learned over the years when developing an interface because we're approaching it from the lens of the idea, which clouds some seemingly common-sense solutions or alternatives.