Thanks for the question. Google Maps Navigation's voice instructions are given automatically based on your current heading and speed and aren't able to be triggered manually at this time. That said - the team's looking into ways to improve the navigation experience, and I can say the ability to repeat a voice instruction is definitely on the radar.
i think this is the only missing feature.
it's simply a matter of having a button you could press by the top of the screen- beside next turn
on my magellan unit this is possible and vzw's vz nav allows this too, i believe.
gnav is a much stronger gps than vz nav though- better nav & super searching
other than that i can't think of another thing gnav is missing.
- the problem is our car's aren't always quiet for example when you have a screaming kid in the back seat and you don't hear sister girl speaking.
business trips are usually quiet, personal ones aren't always.