Slower, prone to viruses, needs defragmentation, developers and designers just seem to do whatever they like and don't listen to the users, a lot of bugs, relatively far more crashes, no centralized application structure, less options for customization without violating the T&C, less potential to learn about how it actually works and, oh i don't know, the fact that you're paying good money for all this? Need i go on? I just can't justify using it, and when i have to in education or when i'm helping somebody else out with a problem (that 90% of the time wouldn't have occurred under Linux) frankly it just gets on my nerves.