From PostgreSQL Wiki

The most authoritative way to find out what has changed in between PostgreSQL versions is to read the Release Notes.

A summary of major changes in features is at Feature Matrix.

8.3 Changes

One of the biggest migration concerns the removal of implicit casts has broken many applications. While ultimately these programs should be rewritten to correct this incompatibility (samples), it's possible to add back in missing casts to ease migration to 8.3. See Re-adding implicit casts in PostgreSQL 8.3 for scripts

Here are a list of articles about new features in 8.3:

• PostgreSQL 8.3 Cheat Sheet Overview
• Synchronized Scans
• PostgreSQL 8.3 Features: Arrays of Compound Types
• PostgreSQL 8.3 Features: Plan Invalidation
• Optimizing function calls in PostgreSQL 8.3
• PostgreSQL 8.3 Features: Enum Datatype
• work_mem tuning and log_temp_files
• Converting tsearch2 to 8.3 (also note the manual section Migration from Pre-8.3 Text Search)
• Integrated Security in PostgreSQL 8.3
• Checkpoints and the Background Writer: PostgreSQL 8.3 Improvements and Migration
• PostgreSQL Performance Features in 8.3
• Upgrading to 8.3 - MediaWiki lessons learned (talks about implicit casts and tsearch migration)

• Source material: PostgreSQL Blog's 8.3 Feature Round-Up