IMHO, that will depend on the type of business, but getting basic understanding of sales (like, actual deal closing), marketing, and hiring are crucial no matter what.

Even if you plan to bring co-founders on board, understanding the basics of what they do goes a long way establishing trust.

Another thing, which is not a skill, is to understand that techcrunch is not a fair representation of the tech world (& neither is HN), and that building a business is not about making the frontpage, it is about bringing solutions to problems so painful that people are willing to pay you for it.

Book-keeping, marketing, and managing high performance teams.