I hate the Skype client for the Mac. It tries to take over everything; adding hooks into AddressBook, who knows where else, huge screen footprint (it's saying "Look at me! Look at me!").
It's interesting that it isn't available from the Mac App Store. Too intrusive for the current set of App Store rules, maybe? So maybe the limitations the App Store puts on apps have some benefit? It will be interesting to see if Skype comes up with a version of their client that meets the App Store rules.