Aaah a short list why Microsux have such a bad product... (maybe i'll post the long list next)
Devastating Windows rot (might be solved in future Windows releases if developers switch from Win32 to UWP).
No enforced file system and registry hierarchy (I have yet to find a single serious application which can uninstall itself cleanly and fully). The $USER directory in Windows, specially in Windows 10, is an inexplicable mess.
svchost.exe (the whole philosophy of preserving RAM this way became outdated years ago).
No true safe mode (rogue applications may easily run in it).
No clean state (for most OEM installations out there). This will be finally solved in new Windows 10 builds.
The user as a system administrator (thus viruses/malware - most users don't and won't understand UAC warnings).
No good packaging mechanism (MSI is way too fragile).
No system-wide update mechanism (which includes third party software - to be fair there are third party applications which offer this functionality, but then such applications don't support core Windows updates).
In certain cases it's extremely difficult to find or update drivers for your hardware devices (anyone who's tried to install a fresh Windows onto their laptop will testify).
Windows is extremely difficult to debug (e.g. try finding out why your system is slow to boot).
Windows boot problems are too often fatal and unsolvable unless you reinstall from scratch.
Windows is hardware dependent (especially when running from UEFI).
Windows updates are terribly unreliable, very slow (to install) and they also waste disk space. The only released Windows 7 SP1 cumulative update, totally breaks the Windows Updates service (the worst piece of software in human history).
And Microsoft did not even apologize for it. Literally billions of watts of energy are spent and they don't give a damn! Did Microsoft fire everyone in their QA/QC department?
Windows cannot replace system DLLs on the fly and restart corresponding services which depend on said DLLs due to its architecture. As a result some system updates require multiple reboots (innocuous malevolence in me requires to mention that in Linux you can even update the kernel on the fly).
Windows keeps trying to reinstall failed updates over and over (in certain cases every such cycle of "updating" can render your PC disabled for hours!).
There's no way to cleanly upgrade your system (there will be thousands of leftovers), etc.
The Windows OS installer doesn't give a damn about other OSes installed on your PC and it always overwrites the MBR. In case of already existing Windows installations, it sets the newly installed Windows as the default OS - no questions asked. In case of UEFI, booting of other non-Windows OSes is unsupported and Windows actively prevents this.
WinSxS, though a neat idea, turned into some madness: Windows keeps the versions of files the user won't ever need: for instance the English version of Windows will have copies of files for many other languages irrespectively of the chosen locale or MUI.
Cryptic error messages (considering the size of the OS, >9GB as of Windows 10, this practice is simply ridiculous).
Most malware writers target Windows as the most popular desktop OS, so it has the biggest number of viruses among all other OSes (over five thousand new viruses daily).
Windows loves thrashing your HDD.
Microsoft programmers are still unable to cope with NTFS fragmentation twenty-five years after its introduction. To make things worse most Windows applications do not preallocate files thus they contribute to fragmentation even more.
Windows anti-virus products oftentimes make your PC less safe - so if you want perfect security and privacy, stop using Windows and migrate to Linux right away. OEM updaters make your PC wide open for attacks.
Microsoft has recently decided that you will no longer be able to download certain Windows updates manually. You'll only be able to get them via Windows Update.
"sfc /scannow" is offered as a solution to most Windows Update Service and Microsoft Installer Service errors, yet in 95% of cases it's totally ineffective.
Windows does not allow you to use any partitions other than the first one on your removable USB flash drive. There's no logic or explanation behind this totally ridiculous and artificial limitation.
Windows does not automatically clean temporary files ever, however it must do that for every reboot/power cycle.
The generic drivers Windows comes with are not always compatible with the wide range of existing hardware. Since Windows has a habit of replacing your vendor's drivers with its own newer drivers your hardware may stop working correctly after upgrading to a newer Windows release (remember that Windows 10 is a codename for major different operating systems, e.g. like Windows Vista vs Windows 7 in the past).
Microsoft has gone crazy: Windows 10 is now a recommended update for all Windows 7/8.1 users unless you're running their Enterprise versions. That means your computer will automatically update to Windows 10 unless you either disable the Windows Update service completely or set Windows updates to the manual mode.
In May 2016 Microsoft started deceptively updating users' PCs to Windows 10 regardless of your Automatic Windows Updates settings or the way you interact with the GWX application.