I don't know, as a recent arrival from Java I sure miss my IDE.

- Having a background parser running all the time means I never save  
a file with a syntax error.
- It's totally silly that I have to type method names for things like  
File and IO and so on, the system should guess them for me
- Refactoring!  I find that I want to change the names of classes and  
fields and methods quite often, and having the IDE do it is a big  
time-saver.
- In NetBeans there's this window on the side (I'm sure Eclipse has  
the same thing) called the "Navigator", which is just a list of all  
the methods and fields in the file you're editing, you click on any  
of them to go there; huge time-saver.
- One-click to run the tests for whatever the cursor's on
- One-click to find all usages of whatever the cursor's on
- One-click to open the docs for whatever the cursor's on
- scope-sensitive select.  Expand scope to current block, next  
enclosing scope, next, next, etc

Yeah, you can do something in Ruby in a third the lines of code that  
Java would take.  But you might be able to write the Java about as  
fast, with a good IDE.

[Oh, btw, I know that dynamic typing makes some of these things a lot  
harder than for statically-typed languages like Java.  Doesn't mean  
they wouldn't be helpful.]

  -Tim