Voice for Steam players defaults to open mic. This causes most people to either insta-mute everyone or just turn it off entirely because the overwhelming majority of the time nothing of any value comes through the mic, unless you like hearing other people's heavy breathing and sound effects. Most people seem to be unaware that their microphone is active.

I would like to direct your attention to this thread:

http://forums.steampowered.com/forums/showthread.php?t=2028994

I would suggest that voice by PTT by default, with an option to switch to voice-activated.