This release includes:
- Better error trapping during installation and upgrade for various incompatible systems, based on error reports from folks newly trying out Drupal 7 on various environments. We think we've caught all the ones we have so far, but please keep the reports coming!
- Better support for exportables in the form of a new Form API type "machine_name" and format IDs stored as strings rather than integers.
- Links can now participate in the D7 AJAX framework, which was previously limited to forms.
- Raised minimum version of PHP to PHP 5.2.4, to facilitate stream wrapper security.
- Numerous tweaks to basic navigation in the default profile, to prevent information overload.
- Fixed a regression in Schema API that prevented modules such as Date from adding to the column types supported by core.
- Profile module no longer shows up on the module listing on new sites, in favour of fieldable user entities.
- Fixing of numerous Bartik and Seven theme style & RTL bugs. There are still lots more, so please help get Drupal 7's new look and feel as polished as possible!
Source : http://drupal.org/drupal-7.0-beta2