The difference is going to be the factory. Anything made in china is going to be built with higher tolerances since the companies can afford it. We here in the US demand more money for the same jobs so the US made companies usually involve more hands on techniques and the build is different. If you take a real close look at like a K2 and a NS or Mervin, you'll see a diffence in how it's built. Other differences are who each company chooses to buy materials from. Glass makes a diffence. There is weak fiberglass, average, and good shit out there. Depends on whose using what. There is also pre-preg resin vs mix on site, wet lam vs what they do in China... Without looking, I can tell when I step on a Ride, or a Mervin, or a NS... the way they're built makes them feel different.
^ What he said.
Also, many boards are being made in either the GST or Elan factories in Austria as well (Rome included). Most times it is more about specific details in a board than the overall construction. Recently I find that the "cheap" boards are not really so cheap anymore and actually perform on par with most expensive boards. It used to be that you would pay more for a board because it had triax fiberglass instead of biax. Though this is still somewhat true, most riders are realizing that just because a board is more expensive doesn't mean it's better. It's all about preference, and what you want out of a board.