Overall, Drupal 7 has improved a lot in terms of core and UI. It is a lot nicer to navigate around the administration area and easier to find what you need. It has a bit of WordPress feeling to it.
From the development aspect, Drupal 7 is better organized and simplified with a few great additions. For example, one core addition is the out-of-the-box jQuery library, hence, the fancy overlay administration interface added in D6. Another cool addition is having the CCK Fields module included.