• It will not work when JavaScript is turned off. The fallback should be a CSS file that comes close to what the JSON stylesheet does. Maybe some tool can create the CSS file from the JSON stylesheet automatically.
• If you are using an editor like Dreamweaver, auto complete features do not work with CSS written as JSON objects. Again, there can be a tool that takes in CSS (which can be created easily with your favourite editor) and creates the JSON stylesheet which can be used to add additional logic or complexity.

Is it really necessary to push styles onto JavaScript? Imagine a photo blogging template that changes its theme depending on the photo being displayed (I believe such themes already exist). Such a theme can be easily implemented using JSON stylesheets. We just need more tools around this idea for us to start using it instead of plain old CSS.