Two really nice things that only AOR and Icom seem to address.
A well buffered IF output, something below 30 MHz and wide banded...in the area of 250kHz or more bandwidth would be ideal.
Discriminator output, and while we're talking audio output, use isolation transformers on them so ground hums aren't an issue for people connecting them all over for processing or recording.
Another suggestion, when possible, don't board mount interfaces like DC supply jacks (12V btw is very convenient for hams) or selector controls and audio jacks. This way they are easily replaced and any stress on them from use isn't wearing solder joints on a board trace.