We're not a YC startup, but we found a designer who doesn't typically do web sites -- he specialized in storefronts, t-shirts, skateboards.. In other words, someone who didn't have any predefined notion of what to do on the web, someone whose core skill is to get a message across in milliseconds and to draw people in.
It worked out really, really well for us; it may be worth your while to look outside typical web designers. That said, we already knew what the application flow would be like, so it was a matter of 'skinning' for us -- color scheme, iconography, etc.