Currently, Siri is just a question-answer machine. While it's very good at this, a personal assistant needs to be more than that. I think the next stage in the evolution is tapping on your personal information, having a memory, and being able to have a contextual dialog. All this should be implemented fluidly.

Example:

Me: "Siri, what's up today?"

Siri: "You should have lunch with Sarah at 5 p.m"

Me: "Make a reservation at that Chinese restaurant which we were in last time"

Siri: "Ok."