cont Lol
You've got no real control over crucial features of the OS:
Windows 10 will have no service packs which means it will always be a work in progress and you are a perpetual beta tester.
Forced updates you cannot opt out of (and Microsoft have borked quite a lot of them recently so prepare to see your Windows die after installing a new batch of updates - actually Microsoft has already borked one update, read horror stories about KB3081424). In December 2016 Chris Capossela, chief marketing officer at Microsoft, admitted that the company had gone too far with the way Windows updates are distributed.
Safe Mode has become impossible to access unless you've booted into ... the running OS, which totally defeats its purpose. Also Safe Mode is hidden behind almost a dozen of steps vs. a single F8 key press on boot in every Windows version from 95 to 7.
Windows 10 anniversary update makes it very difficult (read impossible for average users) to disable Cortana.
Windows 10 violates basic networking principles: it ignores the hosts files, the DNS protocol and firewall rules and sends telemetry data regardless.
Microsoft says that there will be at least two service updates (or whatever their names are) for Windows every year, and each update is basically a new version of Windows, so:
Twice per year you may reinstall software deemed not required by Microsoft.
Some features you grew dependent on will be removed without providing any alternatives.
Your preferences will be reset to default, so you'll need to go through them regularly.
Some Metro applications will be reinstalled if you deleted them previously. New wonderful Metro applications will be installed.
Expect your group policy settings and tweaks to be completely removed or changed and the only way to get them back is to upgrade to Windows 10 Enterprise.
Certain software titles and drivers will cease to work.
Windows 10 features terrible UI inconsistency, not limited to: [block]Windows Evolution. Click to view the full image
Two kinds of font antialiasing (ClearType v2 for classic applications and some awful dirty grayish something for Modern apps).
Truth to be told it's not a problem with HiDPI monitors but few people own them.
All kinds of varying visual decorations and styles (some people have discovered up to seven varying styles in Windows 10).
Absolutely dissimilar classic and modern (PC settings) control panels.
Different font faces and sizes all around.
Different styles of settings for modern apps.
Absolutely different context menus and their appearance in different applications and apps.
Terrible hardly-configurable appearance, dubious design choices and extremely limited functionality (vs Windows 7/XP):
Two Control Panels with absolutely zero thought given to how they differ and why each one should be used.
Some Control Widgets are spread between the two Control Panels which is utterly confusing (e.g. User Management).
No Windows classic UI for windows decorations. Windows decorations can hardly be configured at all in Windows 10.
An awful choice of colors/palette.
Absolutely awful, childish and amateurish icons (the current release features slightly better icons) as if we live in the era of eight-bit displays (only rivalled by those in Windows 3.1 from 1992). Windows 2000 in 1999 looked better than Windows 10 in 2015.
A big number of Windows 10 apps are still NOT on par with their classical counterparts from Windows 7/Vista/XP (many features are missing or many options are not configurable).
The start menu is an unusable abomination. Sometimes classic Win32 applications are not listed anywhere. Applications are listed as a list which is nigh impossible to scroll.
Windows 10 sucks terribly if you are an unlucky user of a metered Internet connection:
It features huge mandatory system and apps updates (you cannot disable them, you can only postpone the system reboot after their installation). A note for smug commentators: certain home users/companies use a 3G uplink connection (for instance via a router which supports USB 3G modems), which means Windows 10 doesn't and cannot know how it's connected to the Internet.
As if it wasn't enough, Windows 10 gets downloaded automatically if you run Windows 7 or 8.1. We are talking about 3-6 gigabytes of data some people absolutely do not need.
It uses your free bandwidth to distribute updates to other users nearby you.
Windows 10 Pro edition has become more or less unsuitable for small enterprises because Windows 10 anniversary update removes the ability to disable the following "features" (more like annoyances):
Microsoft Consumer Experience: personalized recommendations or, in simple terms, ads in the start menu.
Windows Tips.
The Lock Screen.
Disable all apps from the Windows Store.
To disable these "features" you will have to purchase a subscription for Windows 10 Enterprise or Education editions.
Interesting (read awful) features for developers and power users:
Visual Studio 2015 C++ compiler secretly inserts telemetry code into binaries.
Windows 10 anniversary update blocks all drivers which are not signed by Microsoft. At the moment the ext2fsd driver and VirtualBox will cease to function.
In Windows 10 certain not-so-old games and applications either do not work or have severe problems.
Windows 10 shows full-screen ads on your lock screen.
March 2017 update: Windows Explorer now shows ads for OneDrive.
A new shocker: Windows 10 installs apps behind your back without your approval. The first Windows anniversary update without asking first reinstalls Skype and auto-logins you.
Windows 10 resets your default applications to built-in Microsoft's ones after each major update.
As has already been mentioned, different Windows 10 releases are different operating systems altogether, so Microsoft is "free" to deprecate the support for your hardware even if it came with Windows 10 preinstalled. So, Microsoft decided it no longer wants to allow new Windows releases on PCs having an Intel Clover Trail Atom CPU inside.
With Wi-Fi sense enabled anyone you have in your Skype, Outlook or Hotmail contacts lists — and any of your Facebook friends - can be granted automatic access to your Wi-Fi network as long as they're within range.
A newly-created user profile weighs over 300MB (!) while containing zero (!) information about the user.
A newly-created user profile is populated with all the default apps instead of giving the user a choice.