What happens when I reserve?
When you reserve, you can confirm your device is compatible with Windows 10. Between reservation and when your upgrade is ready, the files you need for the upgrade will be downloaded to your PC to make the final installation go more quickly. Then, when your upgrade is ready after July 29, 2015, you get a notification that lets you get started with your upgrade.
How long does it take?
Plan for about an hour for the installation. Newer devices may only take 20 minutes, and older devices may take more than an hour.
You’ll need to download the installation files before you begin. If you reserve, we’ll take care of downloading those files for you and let you know when you are ready to get started.
They are also doing it step wise to make sure all reserved devices can actually have a good "OOBE". Just because, for example, someone has an i7, SSD, and a GeForce GTX 980 Ti, doesn't necessarily mean Win 10 can install well.
Lots and lots of variables to contend with. Upgrade from Windows 7 or 8.1? MSE or Defender or other? Drivers, installed software, peripherals connected? Possibly Pirated or infected with malware? The list goes on. Recovery options in case things go south? This is why they have been pushing us to upgrade instead of doing clean installs. This is why ISO's are few and far between.
Yup to those variables, that's why even though I had absolutely no problems with the Insider upgrades with everything being pretty much left(except for RAPID Mode) I've still cleaned house. For example already deactivated WMC & Gadgets in Win7 and will uninstall MSE a day before. On both machines got rid of all traces of UXStyles and set my Disk properties to Windows default and ran SFC and ran winsat to let windows set to defaults. I want so little carry over cruft as possible.
MSE should be fine, its the others, like Norton, that's almost impossible to uninstall, that could be issues.