If you try to build someone else's idea the passion will be hard to find. Your idea, your passion will be what keeps you up at midnight when you would rather head off to bed.
Having been hired by a few entrepreneurs as "the business guy", and having brought on a few true technical co-founders, I've experienced this nearly every time...it's hard to find passion for someone else's idea.
That said, I'm not convinced it's a bad thing. Just something to be aware of.
Even though my technical co-founders weren't truly passionate about the space(s) we were in, they quickly got passionate about some of the technical problems we were solving. And even though I was never as passionate about the space as the domain-expert founders I was working with, I love getting ideas off the ground and in the hands of paying customers.
To further @keithwarren passion-exploration post, if money was no object, what would you spend your time doing?
That's probably the best question I ever heard from a mentor and the reason I'm in the space I'm in today (using technology to democratize philanthropy).