I would have to say I'm not a great fan of the default skin, it isn't obvious what it is nor is it obvious what things are for or how to select and run a scan at first. This can be a little confusing (perhaps daunting) for a novice user, but it doesn't take long to master. Then the world is their oyster for a large selection of skins.
The greatest issue for Alwil is that not all users will have powerful graphics or high resolution monitors or the latest OS. There is such a large number of permutations of system, OS, graphics, Monitors, that you have to have something that will work with the lowest common denominator and the current default does that.
I'm currently using the Silhouette_4in1 by Sasha which I think is relatively clean and professional, I personally don't really like the no skin option, it too has graphics issues with users in safe mode and 640X480 default resolution in earlier windows versions as it has a 800X600 background image and no scroll bars.