It's not simply about compression, Skype has a decade-long head start on tweaked echo/feedback suppression, gain control, dejitter, adaptive quality adjustment and so on. The call quality is marvelous, even with Macbook mic and lossy hotel wifi (I spend at least an hour every day using it right now).
I'm sure the browser vendors will catch up, but haven't even seen mention of some of these features yet with regard to WebRTC.
My experience is that Mumble blows Skype out of the water for background noise suppression. I can't have a call in a windy area without riding the mute button on Skype, but Mumble lets me adjust the signal/noise threshold to the point where it only picks up my voice.
The fact that it doesn't crash every other day is a nice bonus too.
No offense, and this isn't likely to be popular given the downvote above... but look harder. These are things that at they very least ARE being considered already for WebRTC.
I promise I'm not trying to be abrasive. I'll update with some links when I get a chance to get to a proper computer.
I'm sure the browser vendors will catch up, but haven't even seen mention of some of these features yet with regard to WebRTC.