- Uses intentionally confusing UI to trick users into installing ("upgrade now or upgrade later?" - note the absence of "do not upgrade")
- IMSHNO installs without user consent (closing a "upgrade now?" window counts as "no" anywhere, including MSFT UX Guidelines, hello there hypocrisy)
- breaks existing applications (not every box is just Firefox+LibreOffice: specialized apps and drivers do not work well in WinX)
- while the downgrade ("better to worse" is never an upgrade) runs, you can't use your device ("preparing for blahblah, wait.")
- erases (or at best hides) user data ("yeah well we just put it all in this completely different folder without notice, it's not like you need your documents, right?")
- is very difficult to revert
- phones home (again, MS claims there is user consent. So do all the malware writers; I don't think so.)
Each and every of these are traits commonly associated with malware; thank God for GWX Control Panel.
(To be fair, pre-installed Windows 10 is usable, better than Vista even - but breaking working installs of W7 for no obvious reason is a big no-no)