I don't know how quickly the differences will assert themselves in an existing group which is transitioning from one role to the other, but having worked as both an internal developer and a product developer, two huge differences leap to mind: requirements and and testing.
As a developer of internal tools, I was pretty much given free reign regarding interface, organization, and even scope. Specs were in the form of an email saying "can you write something that does X?" Similarly, testing was almost non-existent. After whatever testing I was able to do, the tools would be deployed directly to their target audience and bug reports, when they came at all, were directly from those end-users, again usually via email or even hall-tackle.
Now that I'm doing product development, the difference is dramatic. Specs are 30-100 page Word documents and we have a dedicated testing department which makes darn sure that what we produce matches those specs. I'm much better supported by the project managers and have clear channels for any feedback I have on requirements or design. It could be argued that product development offers less freedom to the individual developer, but in exchange for being part of a (hopefully) better-organized and better-supported team.