Well, the podcast I used to cohost with a friend, I knew she was the right person to collaborate with a podcast on because she had incredibly witty banter and was a great compliment to the way I interviewed.
You can get an idea of what that sounded like in action here: https://itunes.apple.com/us/podcast/chat-to-the-future/id1181623128?mt=2
She and I still quote the podcast to this day!
Same with another potential podcast host that I wanted to launch a series with, but unfortunately we weren't able to get liftoff - we both brought a very different "style" to the podcast.
I think you want to look for mutually complimentary skills, and strong rapport. If your show is a success, or develops a strong fanbase, you're going to be recording with this person for years (in the case of Keith and the Girl, they've been recording since 2005!) so you want to have good chemistry.