If you've paired a Bluetooth audio device with your Mac, even if it supports A2DP, you might have noticed how awful and rough the sound quality is. I've always put this down to Bluetooth being a rubbish standard when it comes to sound, but I noticed how good a set of Motorokr Bluetooth headphones sounded when I connected them to my Blackberry tonight, even though they sound awful on the Mac.
Turns out that OS X uses a horribly low bit rate for Bluetooth audio by default — who knows why. Perhaps it's to allow for maximum compatibility with all devices.
The easy way to fix it is to copy and paste this line into your terminal:
defaults write com.apple.BluetoothAudioAgent "Apple Bitpool Min (editable)" 40
The "40" is the quality — depending on your Bluetooth headset, you can adjust it up for higher quality, or down if you have connection problems. 40 worked for me with a Motorokr S9 headset.