Downsides of Windows

Problems Windows has that Linux doesn't.

Reboots
Whether you have installed or reinstalled something, updated a driver, trying to get programs running again, trying to get your system to fast again after it slowed down a lot or are just trying to figure out a problem on Windows reboots are a waste of your time and effort.

On Linux reboots are only ever needed if the kernel of the operating system has had a security update, all other are updates applied without rebooting.

Crashes
You might have no idea what caused your Windows session to crash, but you still have to reboot your computer and restart all your programs.

Linux pretty much never crashes, even under severe loads.

Slow-downs
Windows might slow down because you have multiple programs running, or it's over-run with worms, viruses, spy-ware or other malware, or just because it's been running continuously too long.

Linux doesn't slow down no matter how many programs you have running and never gets infected with viruses, worms, spyware or other malware, and doesn't slow down no matter how long you leave it running (running several months

Reinstallations
Windows users reinstall to fix problems on their systems, to get hardware or programs running, or to remove spyware or other malware, or to get back needed speed or harddisk space

Linux never requires reinstallations to fix problems (it generally doesn't have problems) and some editions (distributions) of Linux don't even need to be reinstalled to upgrade to the next version!

Change of interface
Microsoft will change the user interface of Windows to something you don't like, for example the Metro interface on Windows 8, and is not suitable for desktop computers.

Linux will always have a desktop. (Actually it has several. Good ones, too)

Malicously-intended software
Windows setups are often infected with various types of malware, viruses, worms, backdoors, trojan horses, spyware, ransomware and adware, and often many of the above at the same time. Over all there have been billions of malware infections on Windows computers.

There are over 35 000 variants of viruses for Windows. There are 37 for Linux (and most of them don't work due to security enhancements).

Break-ins
Linux is immune to break-ins on a client-configured machine

Registry problems
Windows has wasted harddisk space and slow-downs due to extra data stored in the registry, also system problems can reside there

Linux does not have a registry and so does not have these problems. (Linux uses text files for configuration most of the time.)

Cost
Windows is costly, especially if you have to pay for it twice to get a copy you can set up with the options you like.

Though there are editions of Linux you can pay for, Linux is free mainly, always has been, and always will be.

Extra costs
Windows users often buy firewalls, spyware removal tools, anti-virus scanners or system diagnosis or repair utilities, greatly adding to the original purchase price.

Linux has a firewall, it does not need spyware removal tools or virus scanners and comes with a lot of diagnostic and repair utilities for free.

Even more costs
Windows users often have to pay for repairs to their computer or "cleaned" because the system got slow or infected with viruses.

Linux basically never breaks down and does not need to be "cleaned" because it never gets infected with malware.

Safety
Windows gets broken into all the time

Linux is very safe and basically never gets broken into by malware or computer criminals

Extremely annoying Windows upgrade recommendation
Windows versions seem to have a Get Windows 10 update that annoyingly pops up that obnoxiously doesn't let a user send a clear "no" and sometimes outrageously downloads 6GB of data without permission

Linux will never have annoying pop-ups for operating system upgrades or for anything else, even if you run an old version of Linux

Abandonment
Microsoft abandons old versions of Windows, forcing you to buy a new computer (and a new copy of Windows) in order for you to run an operating system with security updates.

Linux has free updates and will continue to be able to run on your old computer, probably no matter how old it gets.