I use Siri all the time in the car and I find it invaluable. Here are some of my suggestions for improvement:
Continuous listening would be great to have. Maybe you could activate it by speaking "continuous on" and turn it off with "continuous off". Whilst its active you would prefix every command with "Siri.."
It would also be good to have an option to extend how long she waits for you to finish. Sometimes I'm sending a text message and I stop to think about what I want to say. Siri assumes I'm done and composes the unfinished message. An elegant and natural solution would be if you said "ummm..." or "aaah..." she would realise you were thinking and would keep listening until you paused for a longer time or said something like "ok, I'm done".
Another suggestion would be the ability to correct her. After she composes what she thought you said, you could simply say something like, "Not bears, beers"
Every night when I drive home from work I ask her to "call my wife". Then I have to manually press the speakerphone button. I'd like to be able to say, "call my wife on speakerphone"