Since Apple stripped Mac OS X down to just what was required to operate the iPhone, whether or not to call it "Mac OS X" or something "lighter" was a marketing decision, but didn't change the facts. As long as Apple controls all applications, Apple will include just as much of the core facilities as it needs for those apps. So as consumers of this first model, we shouldn't really care if it's "light" or not; it does what it advertises.
The difference matters more when we think about future development. If they open up to other developers, the demand to restore other OS facilities will grow. It's also possible that Apple would feel they need to limit their own development to avoid having to restore what was previously removed, so they have a conflict of interest: keeping the OS small while adding new apps, and that could put a drag on development and innovation.
Luckily, as time goes on, they will be able to increase the amount of flash memory and beef up the operating system, simply because memory prices fall. I hope that will let them keep ahead of application demands.
The difference matters more when we think about future development. If they open up to other developers, the demand to restore other OS facilities will grow. It's also possible that Apple would feel they need to limit their own development to avoid having to restore what was previously removed, so they have a conflict of interest: keeping the OS small while adding new apps, and that could put a drag on development and innovation.
Luckily, as time goes on, they will be able to increase the amount of flash memory and beef up the operating system, simply because memory prices fall. I hope that will let them keep ahead of application demands.