Simple:Press Forum is rather feature rich and to list everything would result in a very long list.
So below I have tried to concentrate on just the main features.

• Supports WordPress version 2.04 to the latest 2.5.1
• Fully integrated with WordPress user registrations and logins
• Installs on WordPress MU
• Creates ‘pretty/seo friendly’ permalinks
• Underpinned by a powerful user and permission system
• Can create private forums
• Forums can be ‘Members Only’ or can allow for Guests
• Private Messaging system
• Admins can allow for post moderation (approval)
• Build in Search tool
• Fully customisable skins and icons
• Most rendering functions are ‘pluggable’
• Member Profile updating and can change own passwords
• Supports Page Password
• Choice of Editors (TinyMCE or QuickTag)
• Options Signatures, Signature Image, Avatars, and Rankings
• A variery of Template Tags - recent posts etc
• Progammable Hooks for user extension
• Optional Spam Prevention tools
• Optional Cookies to track Guests and Members
• Fully Localised for language support
• Optional and various RSS feeds and email subscription to topics
• Various display options to suit your needs and theme
• An Admin toolset to perform many tasks from within the forum itself
• Blog Posts and Forum Topics can be linked
• Support for the Democracy Polls plugin
• Support for the Gravatars 2 plugin
• Forum Statistics - incuding who is online
• Complete uninstall/removal option