After many years of development, PostgreSQL has become feature-complete in many areas. This release shows a targeted approach to adding features (e.g., authentication, monitoring, space reuse), and adds capabilities defined in the later SQL standards. The major areas of enhancement are:
Windowing Functions
Common Table Expressions and Recursive Queries
Default and variadic parameters for functions
Parallel Restore
Column Permissions
Per-database locale settings
Improved hash indexes
Improved join performance for EXISTS and NOT EXISTS queries
Easier-to-use Warm Standby
Automatic sizing of the Free Space Map
Visibility Map (greatly reduces vacuum overhead for slowly-changing tables)
Version-aware psql (backslash commands work against older servers)
Support SSL certificates for user authentication
Per-function runtime statistics
Easy editing of functions in psql
New contrib modules: pg_stat_statements, auto_explain, citext, btree_gin

See the official release notes for a description of changes: http://www.postgresql.org/docs/8.2/static/release-8-2-4.html