We are strictily talking about design, not about CTR or tech.
So, mainly they say that a good banner shall have: Interactivity, good use of space, compelling imagery, good texts, call to action, branding elements and a bit of humor.
What shouldn´t be on a banner: Lost messages, falses promises, excesive impact and bad colour treatment.
In my opinion, if we talk about sales, I still think the most important thing is the publisher where the Ad goes. Talking about branding is different, that way creativity is much more important.