1. The ability to explain things in simple, non-technical terms, even though the issues are usually neither of that. My company builds really cool things -- I need to sell those things to keep it alive. Without co-founders who can help me understand the details and speak at intelligently about our technology, I couldn't do half the pitches I do.
2. The willingness to do technical work "beneath them.". It's grand if your partner is a super talented developer, however, in a startup, they won't be able to always work on the exciting stuff due to bandwidth, and you'll need someone who is willing to deign to do some of the not so fun stuff. For example, the more technical of my partners will still pitch in with simple CSS and HTML tweaks when the rest of the team is stretched thin. His willingness to occasionally do BS that he did in his early teens allows us to keep moving forward with happy clients.
3. Patience and politeness! Your technical co-founder needs to have an attitude that makes them approachable and not intimidating. Staff and yourself need to feel comfortable asking them questions, even dumb ones.