New features in Drupal 7

Database:

• Database layer utilizing PHP 5's PDO abstraction layer
• Added query builders for INSERT, UPDATE, DELETE, MERGE, and SELECT queries
• Added support for the SQLite database engine.
• Default to InnoDB engine, rather than MyISAM, on MySQL when available for greater scalability and data integrity. 

Security:  

• Protected cron.php -- cron will only run if the proper key is provided
• Much stronger password hashes
• Rate limited login attempts to prevent brute-force password guessing

Usability:

·         Added contextual links (a.k.a. local tasks) to page elements, such as blocks, nodes, or comments, which allows to perform the most common tasks with a single click only.

·         Improved installer requirements check.

·         Improved support for integration of WYSIWYG editors.

·         Implemented drag-and-drop positioning for input format listings.

·         Implemented drag-and-drop positioning for language listing.

·         Implemented drag-and-drop positioning for poll options.

·         Provided descriptions and human-readable names for user permissions.

·         Removed comment controls for users.

·         Removed display order settings for comment module. Comment display order can now be customized using the Views module.

·         Removed the 'related terms' feature from taxonomy module since this can now be achieved with Field API.

·         Added additional features to the default install profile, and implemented a "slimmed down" install profile designed for developers.

·         Added a built-in, automated cron run feature, which is triggered by site visitors.

·         Added an administrator role which is assigned all permissions for installed modules automatically.

·         Image toolkits are now provided by modules (rather than requiring a manual file copy to the includes directory).

·         Added an edit tab to taxonomy term pages.

·         Redesigned password strength validator.

·         Redesigned the add content type screen.

·         Highlight duplicate URL aliases.

·         Renamed "input formats" to "text formats".

·         Moved text format permissions to the main permissions page.

·         Added configurable ability for users to cancel their own accounts.

·         Added "vertical tabs", a reusable interface component that features automatic summaries and increases usability.

·         Replaced fieldsets on node edit and add pages with vertical tabs. 

Performance:

• Improved performance on uncached page views by loading multiple core objects in a single database query.
• Improved performance for logged-in users by reducing queries for path

alias lookups"

Search:

·         Added support for language-aware searches.

Testing:

·         Added test framework and tests.

Theme system:

·         Removed the Bluemarine, Chameleon and Pushbutton themes. These themes live on as contributed themes

·         Added "Bartik" theme as the default user interface theme.

·         Added "Seven" theme as the default administration interface theme.

·         Added "Stark" theme to make analyzing Drupal's default HTML and CSS easier.

File handling:

• Files are now first class Drupal objects with file_load(), file_save(),
  and file_validate() functions and corresponding hooks.
• Files use PHP stream wrappers to enable support for both public and private files and to support pluggable storage mechanisms and access to remote resources (e.g. S3 storage or Flickr photos).
• Added a field specifically for uploading files, previously provided by
  the contributed module FileField.

Image handling:

• Improved image handling, including better support for add-on image
  libraries.
• Added a field specifically for uploading images, previously provided by the contributed module ImageField.