How does one determine if a given year is a leap year? It's a common belief that if the year is evenly divisible by 4, that it is a leap year. That is true most of the time, but not always.
Boiling down the zillion hours of D7 development into a short list of important changes isn't practical, so consider this a starting point rather than a comprehensive list.
If you're a Drupal developer, you're aware, no doubt, of the coming of D7. One of the exciting changes in D7 is that CCK has been brought into core...somewhat.