Higher sound quality FM/AM radio, that's about it. Should also send along the data like Song name, Artist and such, like the RDS radios do that are found in newer vehicles. Multicasting is kind of cool, where one channel can broadcast news and music, you pick which one you want to listen to without changing the station.
Keep in mind if your new head unit says "HD Ready" it will still require the use of a tuner box that receives the signals and translates them to your head unit. Last I checked those recievers weren't cheap. The kenwood one is almost 400 the alpine almost 300, I would say you would be better off looking into sat radio, as not only will you get better quality sound, but no commercials on music channels.
Oh and here is a list of the stations in MN currently broadcasting in HD:
http://www.ibiquity.com/cgi-bin/list...sort=StnFormat
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