After using many VoIP solutions over the years I have some ideas about what features I want in my VoIP: easy to use, reliable, works from anywhere, behind any NAT, has critical mass so you that you can find many other users on it, free calls between users, presence, IM, video nice, but not really needed in most cases, interoperates with the other solutions out there so you don't all have to be on the same provider. It would also be very nice if it was an open system where anyone could participate, but that sometimes does create quality problems.
Seems to me that Skype is the one closest to filling all of these features. The ones they don't fill are
I have also been thinking about the things that I can only do in Windows: my Nokia N95 software and the Skype USB cordless phone, I wish I had drivers for those 2, they are one of the few things I can't do in Ubuntu these days.