Lets make this short and sweet: the user interface of Skype sucks.
It is too dominating on the screen and it does not take into account how people actually use similar products. If you look at the closest relatives of Skype, the aggregating chat clients, you will see what I mean. Small, focused on being unobtrusive and usually centered around how people actually use communication software. By comparison, Skype wastes screen real-estate and doesn't really show you that much of what you are interested in.
In fact, I was going to include an illustrative screen-capture, but I decided against it. When thinking about why I realized it was because I didn't want a honking big graphic in the middle of my blog posting. That is how badly designed the Skype UI is.
No, I have no use for a cover-flow-like UI to leaf through my contacts. Copying UI design mistakes by Apple and then, to make matters worse, using them in the wrong context is just stupid. Nor do I have any use for seeing a list of whom I talked to two days ago. No really, I don't.
What I have some use for is to see which one of my contacts are online. Without having to click around and then reshuffle windows. This should be the default view. Not some oddball, tucked-on auxiliary UI wart like now.
One more thing...
What really puzzles me is why nobody at Skype cared enough about the product to ensure that people would have a reason to run Skype all the time when logged in. In case you hadn't noticed: most people don't. They fire up Skype when they "need it". Often initiating the conversation out of band via chat, email or (gasp) phone.
The easiest way to accomplish this would be to make sure Skype is also an IM aggregation client. A good IM client. One that is on par with, say, Adium. With support for all the major chat networks. (If you work for Skype and are not familiar with Adium I suggest you install it now and start using it. It is important you understand why Adium is so popular)
You see, a lot of people would like to have Skype support in their IM client. This is no accident. Because they run their IM clients all the time. They do not run Skype all the time. Because it is an annoying piece of single-use software.
By ensuring that Skype is also a great IM aggregation client you would give people real reason to run Skype all the time. Which would boost the usefulness of the application more than any amount of advertising could ever do.