1. Easy record and playback.
2. Intelligent field selection will use IDs, names, or XPath as needed.
3. Autocomplete for all common Selenium commands.
4. Walk through tests.
5. Debug and set breakpoints.
6. Save tests as HTML, Ruby scripts, or any other format.
7. Support for Selenium user-extensions.js file.
8. Option to automatically assert the title of every page.
Advantages: Selenium IDE is the only flavor of Selenium, which allows you to record user action on browser window. It can also record user actions in most of the popular languages like Java, C#, Perl, Ruby etc. This eliminates the need of learning new vendor scripting language. For executing scripts created in these languages, you will need to use Selenium Remote Control. If you do not want to use Remote Control than you will need to create your test scripts in HTML format.
Disadvantages: Biggest drawback of Selenium IDE is its limitation in terms of browser support. Though Selenium scripts can be used for most of the browser and operating system, Scripts written using Selenium IDE can be used for only Firefox browser if it is not used with Selenium RC or Selenium Core.