Who qualifies for the free upgrade?
* Only non-enterprise users with a valid Retail or OEM and Genuine x86/x64 Windows 7, or Windows 8/8.1 Starter/Home/Standard/Core/Pro/Ultimate (whatever you want to call it) license qualify for a free upgrade. This is very simple, end of story.
If you have a non-enterprise version of Windows, that is joined to a Windows domain, you may also not be offered the Free Upgrade automatically. We don't yet know whether the license terms will allow business users with non-enterprise versions to legally upgrade or not, although it's hard to say how legally enforceable such a claim would be anyways. Regardless, if you want to get the free upgrade to your non-enterprise qualifying OS, then you may be forced to unjoin from the Domain, perform the upgrade, then re-join. We don't yet know if a you will get a free upgrade from ISO if you are joined to a domain.
* If you have an Enterprise edition, you do not qualify. If you have Windows RT from an ARM Based tablet, you do not qualify. We don't yet know about more exotic versions that most people have probably never heard of. Any version of Windows 7 or 8/8.1 that is efectively a Retail or OEM will qualify (ie, non-VL MSDN or Technet licenses, etc... even if you are no longer in those programs, so long as the license is valid).