Windows is licensed. This is perhaps the most obvious difference between the open source offerings and the proprietary world. If you want to use Windows in production, you have to ensure that you're correctly licensed, or face a fine for being out of compliance.
Funnily enough, this is probably the biggest reason why Linux saw the adoption that it did. When faced with a free offering of something that can do the exact same thing you're trying to do with Windows, most admins would at least try the free version first.
The second most obvious difference, although this is slowly changing, is the fact that Windows installs a GUI by default, whereas Linux prefers a simple text prompt.