Blaine explains the difference between a Smalltalk environment and something like Eclipse:

I generally look at Smalltalk's IDE as playing with a live patient. You are in the middle of a living and breathing system that reacts immediately to you. There's no shutdown, compile, restart, and retry. It's all happening right here and now. It's an incredibly cool experience especially when you realize that you can execute any code and inspect the state of the system. Eclipse on the other hand is like looking at your objects through a window. You can do some manipulation, but you can't mess around with the guts of the patient like you can in Smalltalk. The thing that truly shocks me is why Ruby and Python developers still try to mimic Eclipse (ie. run the code in a different process) in the IDEs available. There's no IDE for them that allows you to be in the middle of a live system. Why wouldn't you want that? I can't wait till Ruby does have a great IDE like Smalltalks. It turns the amp to 12 instead of a mere 11.

And that really is the difference. Take the server running this blog - it's a headless (i.e., no UI) development image. When I update it, I can load code that modifies existing objects in the system. Need to add an attribute to all the registered users? No problem - I load the code, and every single extant instance gets the new attribute.

The same power exists at development time and and runtime. In the Java sphere, you don't get that power during development, much less at runtime. Which is why I call Java tools - even well regarded ones, like Eclipse - pale shadows.