The biggest diff is just what I get every day at work testing HD vs. SD TV. Analog will play some sound, even if it's white noise, as long as it gets a signal. Digital is silent if it loses signal, and with an intermittent signal it skips like a scratched CD.
Most of the Bay Area HD stations are broadcasting two programs (HD1 and HD2) on the same frequency. They are always different content, sometimes even aimed at a different audience. One of the Spanish channels has English content on HD2.
Edit add: The new radio is more powerful and has better sound quality all around than the old one
Deleted comments to this post and the other post about HD radio because the bozo who started the comment thread keeps a locked LJ with all comments screened and unread - an LJ site set up solely to harrass people who say anything nice about HD radio.