I think the first points are the most salient. Developers undervalue the skill of marketing and sales frequently. It is one of the most powerful secondary skillsets a creator needs. If you are unable to inspire, trust and expertise you will forever have a hard time selling your product. I wrote a post on this area just a couple hours ago and what mindset can help fix that: http://blog.adimofunne.com/no-one-gives-a-shit/