ISSUE: VOICE - sidetone
"Built into the public telephone systems, sidetone is the mixing of a small amount of the microphone signal into the speaker signal. It enables you to hear what you are saying in your own receiver, resulting in a more normal conversation. IP telephony provides no sidetone. This can be very distracting, especially when using a headset."
NOTE: I just purchased a Plantronics DSP 500 headset ... and am VERY DISSATISFIED because it does not provide sidetone. This would not be a problem for Second Life, except that it is a recommended headset.
In any case, the issue of "hearing your own voice" -- not an echo, but the immediate sound of your voice as you do on the telephone (or the headphones in a recording studio, or vocalists on stage) -- should be highlighted in the Second Life voice information.