But the coolest part was how you could add functionality. The framework let you write new commands that interacted well with the parser and gave hints to the autocomplete, and even custom UI with HTML & JS. And then users could subscribe to your commands. Ubiquity would cache the code and periodically, seamlessly upgrade your browser with new versions of the commands you subscribed to.
I was very sad when they discontinued development, and Unity looks like an excellent place to put these ideas to work again.
Unfortunately, my main complaint with Enso and Ubiquity was that both were unfortunately rather buggy. And given Ubuntu's track record with Unity, I suspect that it will be just as buggy.