Simply put, this is a gold mine for custom ROM developers. They will be able to take advantage of all software features, like Smart Stay and S-Voice. Devs are working on it as we speak, so let the fun begin.
We will also be able to extract all those .apk files. People have already extracted S-Voice and it is working with other devices. It does seem to have its fair share of issues, though. It should mostly work with other Samsung smartphones. Users can download it here if they are feeling adventurous.
If you want to download all 786 MB of Galaxy S3 glory, just head over to the XDA Developers thread and have at it! At least to look around until we start seeing it being ported to all other devices.