The sad fact of the matter is most people do not give a hoot about software being open. I honestly do not think you stand a chance competing against Skype, it is so insanely great and wide spread. I'd be impressed if they can actually make the UI/UX better than the latest (horrendous) Skype client. It's like the Diaspora of VoIP.
Everyone I know who uses Skype complains (eventually) about the quality.
Additionally, Skype is effectively a closed (in the sense of opaque) system controlled by corporate interests. Now, I'm not against someone making a fair return on a fair service, but I do worry about who has control over my information. Who is Skype accountable to?