What do you do when you have a great potential co-founder, but don't like their idea?
1) I feel they're thinking too small. They're incredibly talented people, but they want to pursue what I think is a really small market, and could be doing something more worthy of them.
2) We have different opinions on the business prospects for the idea - I think it's just OK, they think it's amazing.
3) The idea seems boring to work on, often because the idea is 95% marketing or content and not technology (e.g. a variant on an online store, blog, content site, etc.) This is bad for motivation.
I have good ideas of my own, but it's hard to persuade people to change their mind once they've decided what they want to work on. Since I'm not in school, I've had to work hard to find other good people (through a lot of networking) and it would be a shame not to do something with them, So, should I move on and look for someone to buy into my own ideas, or start working with these talented friends of mine and hope to evolve their idea into something better?
Finding the right co-founders is much harder than finding the right idea in my experience, so I think that is why the general YC motto is the idea doesn't matter nearly as much as the co-founders do.
Best of luck...
Your course of action, I guess, really depends upon how far along you are though. If you're near a complete beta or working release, you might want to just ride it out.
(1) As has been said many times, having 100% of a small market is more attainable than having 1% of a huge market.
(2) If they think the business prospects are amazing, that's great motivation for them.
(3) Boring is good for startup success. The last field you want to compete in is the one everybody's excited about.
If the idea flops, drop it and do something else. If it works, use it to bootstrap your next idea.
If necessary, come up some criteria you both can agree to.
Remember that you both have to be flexible, or the company won't fly anyway.