I posted an article on Weblog Tools Collection about the top 10 characteristics of a great WordPress plugin.
10. Easy Installation
9. Seamless Upgrade
8. Uninstallation Option
7. Meaningful Error Messages
6. Localization Support
5. Intuitive Admin User Interface
4. Reset/Import/Export Options
3. Optimized
2. Secured
1. Good Support and Thorough Documentation
Read the full article here.