Here is the list of features:

4.7 features

• no module dependency.
• uploading .jpg, .png, and .gif images and previewing.
• option to allow uploading non-image file types.
• support for private downloads.
• limits for file size per upload, total directory size(quota), and image dimensions.
• option to use personal folders or a shared folder for users.
• file sorting according to file name, file size or date.
• highlighting of active files.
• built-in support for TinyMCE

New features in 5.x

• built-in support for FCKeditor.
• built-in support for inline image/file insertion into plain textareas.
• automatic thumbnail creation.
• custom resizing.
• role based settings.
• custom settings for user #1.
• file filtering according to type and extension.
• keyboard shurtcuts (UP, DOWN, DELETE, INSERT).
• javascript API that allows custom usage of the browser, which makes IMCE suitable for any wysiwyg editor.
• admin ability to switch to any user's settings.
• administration of user files in user/x/imce pages.
• sub-directory navigation that allows management of multiple folders.
• absolute and relative path switching.

In order to use new features of 5th version in drupal 4.7, there is a special edition of IMCE. You can try these features without upgrading to drupal 5.0.

New features in 6.x

• Improved UI:
  • Tabbed interface for file operations.
  • Resizable workspaces.
  • Ftp-like directory navigation.
  • Log messages.
  • Additional keyboard shortcuts: home, end, ctrl+A, R(esize), T(humbnails), U(pload)
  • Multiple file selection(using ctrl or shift).
• Ajax file operations.
• Operate on multiple files at a time.
• Directory caching.
• Themable layout using tpl files.
• Improved configuration profiles.
• Multiple personal or shared directory assignments for users.
• Permissions per directory.
• Option to use total user quota together with the directory quota.
• Replace method options for existing files.
• Multiple thumbnail definitions.
• New integration API for wysiwyg editors.