* Great dependency management. The source code of all your Gems is accessible/searchable and always present. If you're using a method in one of your external dependencies, you can 'command-click' through to the Gem source. * Full integration with RSpec/Cucumber. You don't need to break out of your work flow to run a test. You will be presented with a small panel that includes our test results and clickable stack traces of failures. * Good refactoring tools: Extract a variable or method from a code block with a few keystrokes. * Etc.. (the list goes on and on)
Given the nature of the Ruby language, the IDE can only do so much, comparatively speaking. The Java IDEA IDE is arguably more powerful than RubyMine because of the static analysis capabilities of the Java language. I'm not at all saying that I would prefer to develop in Java because of 'better' IDE support, it's quite the opposite really. RubyMine does a lot to increase productivity by making assumptions about how developers will use the tool that result in a very desirable RoR IDE.