Of course, it's not hard, but, it's attention to detail that counts, and that's what takes time. It needs to be right, down to the last pixel. Everything placed just so, in relation to the other parts of the skin. All those options and areas of the program you never visit need to be checked also to ensure that they are OK for those that do use them... more time...
If you're really going to do it right, you don't rely on the stock "Default Skin Items", you create your own graphics for the header menu bar, the tabs, the DSP, Shuffle, Repeat, Display view, Volume options, quick search toolbar, the selection rectangle, full screen player bar....
The menu background has to look right... be original.
Choice of colour is important and has to work in all areas of the xml where they are shared.
I work on the principal that the important thing is the presentation of the media, and the skin should not distract from that, and a hastily made skin does indeed distract. Skins with super high contrasting elements are also distracting, especially if your involved in a marathon hours of tagging. Getting it all to hang together in that way, borrowing as little as possible, takes time, and the less of that you have spare, the longer it takes. It should never be rushed to release. The
2nd Stage skin from Mr Chriz is a prime example of this, released after being in developement for years!
And that first skin... deary me, learning how the matrix drawing works, experimenting with the xml, learning where the colours are shared, that takes almost forever!