What is an example of a specific function or feature that is mandatory on HD DVD but not Blu-ray?
One is advanced audio compression. HD DVD ensures that a Dolby TrueHD track can be decoded to two channels audio, though fortunately the two Toshiba HD DVD players out there go beyond that and support full 5.1 Dolby TrueHD decoding.
Blu-ray did not make that mandatory, which is why
they have to use uncompressed PCM if they want high-fidelity audio. But PCM is very wasteful [in terms of disc space]. Even if you have one hour of silence on a soundtrack, you still have to use the same number of gigabytes to store it.