What clicked about Smalltalk was a few things: one was simply the maturity, both of the community and of the technology. Smalltalk implementations have been refined over the last 25 years, and they’ve really benefited from it.
One of the major benefits, in my opinion, is that Smalltalk VMs are fast enough that it’s reasonable to implement all of the standard libraries – Array, Hash, Thread, and so on – in Smalltalk itself. So there’s no barrier where you switch from your Ruby code to the underlying C implementation; it’s “turtles all the way down”. This may not seem like a big deal but once you have it, it’s really hard to give it up.
In Smalltalk, you don’t have to choose one file layout or order of methods and classes in your code; you’re constantly switching views on what’s basically a database of code.
My friend Brian Marick hates this, because you lose any “narrative” that might have existed by putting the code in a certain order in the file, and it’s hard to get used to, but for me, it’s the ultimate power tool for hacking on a large code-base.