Upgrading to Windows 10 is easy if you're already on Windows 7 or 8. However, if you want a clean install, you have to install an older version first. Next month, the first big update to Windows 10 will fix this.
According to a post on the Windows Insider blog, Microsoft is currently rolling out a new activation method that will allow Windows 10 users to enter a product key from Windows 7, 8, or 8.1 to activate Windows. This should be much easier than the current method which essentially requires you to install Windows twice.
If you're currently running a non-activated version of Windows, or you're waiting to decide whether you should install it, this info is handy to keep in mind. Next month it will be a lot easier to activate Windows on a clean install. If you're currently a Windows Insider, you can check out the new activation in Preview build 10565.
Announcing Windows 10 Insider Preview Build 10565 [Windows Insider Blog]