Source: Gary Leff - View from the Wing
Delta always used to be fantastic for domestic flyers looking to upgrade, but awful for anyone who cared about being bumped up to a better cabin internationally.
Sure, they prioritized full fare over status (which United does now as well, since integrating with Continental) but they do give away unlimited complimentary upgrades and their domestic premium product is above average.
But international upgrades — those required buying a nearly full fare “M” class economy ticket even to be able to play the upgrade lottery. You’d pay about as much as a discounted business class ticket to get on the upgrade ...