Before an effect would target a player and their only defense would be their armor class. This was modified to use a touch attack in the third edition and included my favorite weird wording rule the "ranged touch attack". Depending on the type of activity the target would then get a saving throw.
This sounded simple but could lead to several rolls and table lookups for a single activity.
Fourth edition simplifies this by adding three new defense types to the process. Now an attack might target the characters Armor class or target one of the new defense values Fortitude, Reflexes, or Will. So the same action will now require one role against the targets defense value. This is a small change that will greatly streamline the process while also reducing the accounting required by the DM.
Tomorrow I'm going to take a look at the new format for encounters in the Keep on the Shadowfell adventure.