What's wrong with windows?
Windows is a messy operating system
Operating systems should be designed to do a specific job, that of managing a computer system. They allocate memory to user processes, allocate processor time, and provide various functions to user programs. Operating systems should do as little as possible and do that little very well. Windows doesn't take this approach and instead tries to do everything at one. And it does it badly. For example under every other operating systems a web browser is a user program, it's installed and run by a user and is quite seperate from the central functions of the operating system. Having user programs separated from the operating system allows for the program to misbehave without having serious consequences. The worst thing any user program can do on a well designed operating system is not work. On windows the web browser is built into the operating system, any fault in the web browser ( and there have been many ) can and does compromise the entire system. By 'compromise the system' I mean someone anywhere on the internet can install software on your PC, download your data, or use your machine for spending out viagra spam. On a well designed operating system problems like this can't happen, on windows they happen all the time.
I often hear the phrase 'computer virus'. These two words are always together like they belong together. There is no such thing as a 'computer virus', it's a myth. Computers don't get viruses, Microsoft Windows does. Linux doesn't have viruses, None of the commersial unix varients have viruses, OS X doesn't have viruses. Due to a mixture of bad design and bad implementation windows can become infected by very small computer programs, once infected the computer will become more or less under the control of whoever wrote the virus. I've often heard it said that viruses only affect windows because virus writers pick on the biggest target. This is flawed reasoning. Virus writers pick on the only target that has no effective defence against viruses. At the time of writting it's been estimated that apple OS X has between 5% and 10% of the market share of end user systems. It should therefore be the target of 5% to 10% of the viruses. Currently there is not a single known virus for OS X. Virus infections are caused by bad operating systems and not by evil genius programmers with some hidden magical knowledge.
Windows is unreliable. It's not as bad as it used to be back in the windows NT days but it's still bad. Windows crashes, windows loses data, parts of windows still stop working for no known reason until the machine is rebooted. Most of the unreliability is glossed over by people thinking 'computers are just like that'. Computers are not. There is no reason why computers can't carry out the same tasks for decades without interruption, UNIX derivatives will run for very long periods without trouble. Windows is unreliable because it's poorly implemented and for no other reason. Computers are not 'just like that.'
Related to unreliability is the reboot problem. Almost any change in windows requires a reboot. This is the product of bad design, there is no reason at all why an operating system should have to reboot whenever you install a new program or change a network setting. Other operating systems don't force you to reboot for no reason.
Open standards and proprietary lock-in
Operating systems and user tools and designed to manage data. Microsoft operating systems and user tools are designed to make sure you can never get your data out again. Once you have your data in a Microsoft format Microsoft will do everything they can to make sure you can't convert this to any other format. By doing this they force you to buy upgrade after upgrade when all you want is to manage your own data. Word document formats are designed to be incompatible, excel sheets are designed to be incompatible. This is called proprietary lock-in and it's a nasty trick. Linux won't do this, if you put data in you can get it out again whenever you want.
Windows is expensive. Microsoft products are expensive. Why spend money on bad software when you can get better software for free? It seems nuts to me.
Lack of system management tools
Due to the graphical interface first design of Micrsoft Windows there are a lack of decent tools to manage these systems. It's near impossible to find out what a windows system is really doing at any given time. Other operating systems allow system managers to get reliable information on running processes, windows can't give accurate information. If you can't be sure what a system is doing how can you trust it? I don't think you can.
It's a common feature of operating systems to manage software in packages. These packages allow software to be installed and removed cleanly. Windows doesn't package software in any real sense, it's just not possible to be sure that the system is in the same state after installing then removing a software product. This lack means that installed software may not uninstall at all, or windows may tell you it has uninstalled but in reality it will still be installed and still be running. As having one program installed may prevent another one from working it gets very messy when you can't be sure what really is installed. The normal answer is to reinstall your systems periodicly, it's a very poor answer.
Lack of error messages
Good software returns error messages when it encounters an error. Windows is known for a lack of error messages. I assume this is because most users don't understand them anyway. Windows software often returns messages like 'An error has occured. Please contact your system administrator.' What does that mean to the system administrator? Next to nothing as it doesn't say what caused the error, what the error was, or what program encountered the error. In contrast Linux will normally return a message that will give the system administrator a clue as to where to start looking. Linux and other UNIX systems will also log system messages which can be useful in finding and fixing problems, windows logging is a joke.
'It must be good because everyone uses it'
I have heard this countless times. People seriously claim that windows must be good because lots of people use it. This is flawed reasoning. Windows it widely used because of a mixture of very good marketing, a lack of understanding of technology ( after all it's still pretty new ), and anti-competitive behaviour on the part of Microsoft. Microsoft are estimated to spend 1 billion USD a year on marketing, that kind of money could sell snow to Eskimos..
Footnote - fanatics ( on both sides )
Let me first admit that no operating system I have every used is perfect, they are all imperfect in one way or another. Anyone who states that absolute perfection exists in this imperfect world has lost their sense of balance. Sadly these fanatics turn up everywhere and derail otherwise sensible discussions. Worse than that people who make valid points that are not fully understood are derided as fanatics as an easy argument against them. Fanatics of the world - just give up, you are not helping.