Let users create and share small commands. Create a simple natural language for commands that are easy to program, extend, and remember, and narrow the scope of inputs the voice engine has to deal with.
It was so beautiful and effective and just light years ahead of what we're getting.
Microsoft gets a special mention for lost potential here. Their voice system in Windows could be a way to navigate the layered menus of the OS, but it is mostly focused on answering general queries. Voice is a great replacement for the program launcher, except it's not customizable, but that's about the extent of how much you can control the system with it. Let me do anything buried in the control panel, show me everything you know about a process when I ask, solve that first, then worry later about telling me how big the moon is. You make an OS, don't forget what that is.