Been thinking about this for a few days!
Originally for me it was the cultural icon they are and a time that they take me back to. The more I've collected, however that changed slightly. I went through a LED meters phase where that was my favourite feature, but for the last few months it's been the ability to tune the sound based on the music you're listening to. To tune the bass, treble, graphic eq, loudness, whatever else, to bring the best out of the radio is what I love playing with the most.
Especially something like the 777 or the BigBen that allows for so many little adjustments to really bring out some wonderful sounds. Nothing beats tinkering with these settings I find while going through some favourite tape recordings.
Rock On.