<< whats so great about this? Its just packet writing isn't it? >>
No. Currently, packet writing is proprietary. Every different OS has different, incompatible UDF drivers, and even on the same OS, different manufacturers ship different, incompatible UDF drivers. It's basically a mess of drivers everywhere, and your chances of being able to read a CDRW written on machine X when you take it to machine Y is almost zero.
Mt. Rainier is intended to be a universally compatible UDF specification. It should work seamlessly across Windows, Linux, Mac, and everything in between.