I still can't understand how this gets missed in every one of these posts; a great business cofounder can be someone with an insane level of domain expertise either selling something like the thing you're making, or selling to the exact same people you want to sell to. If you're making, say, web analytics software for online retailers, your best business cofounder would be a guy who had just spent a year convincing o…
More to the topic, a proven sales person is not the same as a business cofounder. Sales experience is a minimum, but with this model it's basically assumed you have a sales ready product.
Domain expertise is great too.. in all hires.... if you can find it. That's where you'll source your "strong-vision guy" if you're lucky. Very lucky.
Marketing (real, user-adoption marketing), PR, design, and perhaps channels to funding and recruiting-- that's what I consider business-side startup execution.
Where do you find these people? I don't know- it's different for devs. People can say, "I can program." It's less cool for people to say, "I can market. I can run a PR campaign." It's not the primary factor, but I think it's a factor.
Signed, Business Guy.