• It dramatically reduces consumer choice.
• It puts Microsoft in a position to dictate prices.
• It makes it very difficult for innovative new companies to break into the marketplace for PC software.
• Microsoft operating systems make heavy demands on computer hardware, which means that one needs a high-spec machine to run them. This means that PCs are rendered prematurely obsolete by Microsoft's need to persuade or coerce customers to upgrade to new versions. The cost implications, especially for smaller companies or those in the developing world, are severe, and it has also been a cause of much concern amongst environmental groups.
• In the extreme, society's ultra-dependence on Microsoft software means that the IT infrastructure of most societies will be critical dependent on a single company which has been convicted in the US courts.
• Microsoft software has a well-documented record of security vulnerabilities. The greater the dependence of any organisation, or society, on a small number of options, the more worrying the the implications of any failings. The 'gene pool' needs diversity.

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The above has been lifted from an essay at livingwithoutmicrosoft.org