Drupal 7

System Requirements

Database: MySQL 5.0.15 or PostgreSQL 8.3

PHP Version 5.2 or higher

PHP Memory: 40M - 64M

jQuery UI is in core now. Before you had to install this library.

Old table-based themes are removed from core. New theme "Stark" was added to the pack, this theme can become a good starting point for you to create new themes and makes analyzing Drupal's default HTML and CSS easier

Administrative interface has got its own theme ("Seven").

IDs of many blocks are changed and has become more meaningful, for example the ID of navigation block is changed from block-user-1 (in Drupal 6)to block-system-navigation(in Drupal 7)– much more easier to understand what the block is for. Some variables are renamed

$picture changed to $user_picture

Some important hooks like hook_page_alter, hook_form_alter, hook_js_alter, hook_css_alter you can use now from theme – no need to create a module

$content became a region and is now mandatory in all themes. In Drupal 6, it was only possible to put blocks after the main page content in this region and to put blocks before the main page content developers needed to create a new region. Drupal 7 now makes the main page content its own block. This makes it possible to put blocks before or after the main page content in the region without hacking in a new region.

CCK,Token, FileField, and Poormanscron into core

Security improvements

Protected cron.php -- cron will only run if the proper key is provided

Implemented much stronger password hashes that are also compatible with the Portable PHP password hashing framework.

New permission to run site updates

Rate limited login attempts to prevent brute-force password guessing, and improved the flood control API to allow variable time windows and identifiers for limiting user access to resources.

Database

Added query builders for INSERT, UPDATE, DELETE, MERGE, and SELECT queries.

Support for master/slave replication, transactions, multi-insert queries, delayed inserts, and other features.

Default to InnoDB engine, rather than MyISAM, on MySQL when available for greater scalability and data integrity.

enabling the use of many databases, such as Maria DB,MS SQL,Oracle,Mongo DB,SQL Lite

Drupal 7 introduces a completely new database API

drupal6

db_query("INSERT INTO {mytable} (intvar, stringvar, floatvar) VALUES (%d, '%s', %f)", 5, 'hello world', 3.14);$id = db_last_insert_id();

drupal7
$id = db_insert('mytable')  ->fields(array(    'intvar' => 5,    'stringvar' => 'hello world',    'floatvar' => 3.14,  ))  ->execute();

Performance Improvements

Improved performance on uncached pages.

When logged in, the Drupal system will perform fewer queries for path alias lookups.

Others

Added support for default text formats to be assigned on a per-role basis.

Improved time zone support

News aggregator: Added OPML import functionality for RSS feeds; Added feed update options.

Installer can be run from the command line

Setting up automated task runs (cron) can now be achieved via Drupal's configuration alone, without having to install any scripts on the web server.
Better Support for Multisite Installations

Added RDF support

Better support for search engine optimization and web linking

Testing module