David Berlind does a good job of listing the remaining glitches with Skype. (Disclosure: He used to be my boss many moons ago.) I still can't get anyone in my family to use it, so I've been mostly using the SkypeOut feature. It works fine 95 percent of the time, but I tend to get "drop outs" at the worst times, such as when I called my mother recently to wish her a happy birthday. That was during peak time in the U.S., so that's probably part of the problem. I've thought of buying a SkypeIN number, but I'm not convinced I can make it pay. No one calls me on plain-old telephone lines, either...
What I'm more concerned about is compatibility with other VoIP standards. When will Skype do a SIP gateway, for example? That could be accomplished (maybe as an another add-on service) without jeapordizing Skype's ability to "just work." The downside, of course, is that such gateways would introduce a whole new set of security concerns.