The thing is that switching to your Desktop feels like an useless operation. You could instead launch an application you are planning to use, because clicking the Desktop first can be seen as an extra click.
(Disclaimer: I have used Windows 8 since the day it came out, never had to click on Desktop; but YMMV may vary [see second section])
See the Start Screen Blog Article for a deeper understanding behind this choice:
The Start screen is not just a replacement for the Start menu—it is designed to be a great launcher and switcher of apps, a place that is alive with notifications, customizable, powerful, and efficient. It brings together a set of solutions that today are disparate and poorly integrated.
The first thing customers will want to do when you boot your computer is launch something, don't you? :)
Here is where the power of secondary tiles come in, you can even pin your favorite messaging contacts.
However, you might be one of those users that has his Desktop full of icons and aren't going to listen to that; hence which is why I do have a solution for you that allows you to go straight to the Desktop:
Place the Desktop in the upper left of your Start Screen.
As the Start Screen supports Keyboard Navigation, the upper left is selected by default.
Just press Enter to launch the upper left entry, this works anytime.
This is of course not automatic, but as the known registry hack might/does not work anymore this is as close as we can get for now. Unless somehow looks into the login behavior and finds a way around.
As there is disagreement on actually having to use Start Screen, one could try to port the Start Menu:
Your best bet is to see whether you can port executable code from the Windows 8 Developer Preview or Windows 7, other than that you'll need to live with the changes.
However, this might come with its own adventure of working around OS security and what not...
Start of by trying to copy
Explorer.exe as well as monitoring/copying its dependencies. You might require an API monitor or debugger to get a deeper understanding on what the Explorer is doing, don't forget to copy the registry settings as well.