UX Booth: Usability Fact vs. Fiction
Usability is user experience
The fundamental difference is "Can easily use" vs. "Want to use". Let's take the best possible example here – the usability guru: Jakob Nielsen's website. Usability guru that he is, his site is very functional and easy to use. But how do you think it scores on user experience? Would visitors love to browse the site for better experience? The answer is no.
Again, there are some practitioners who believe building a good user experience is all that's needed for making the site usable. That had led to the myth in question. Superior user experiences have failed miserably in usability, as it's just one of the dimensions of usability. It is still important to have the right balance.
Usability is good design
Good design (leaving apart the subjective angle) is only an element or output of usability – in practical sense. Otherwise, it's more like a philosophy. Usability done with a philosophy of good design, would make it effective, error-tolerant, engaging, etc. This part I echo. My argument is towards the literal aspect about the myth, that a good design brings usability. In a way, this has a resemblance to Usability is User Experience. A good user interface design is not going to save a product if it is not useful or low on performance or erroneous. The usability and acceptance of such products have no connection with the presence or absence of good design.