The no nesting thing is huge! We refactored a lot of CSS earlier this year and that was one of the biggest pain points. Coupling the structure of your template to your CSS makes changing things very painful. Nesting also creates ambiguous classes that have no obvious connection, consider the following:

.my-component {

    .header {
 
        ...
  
    }  

}

You would just see header floating around in the markup, unsure of where it's namespaced.

Also I disagree that you shouldn't modularize your CSS. We now create a CSS file who's name corresponds with the class name.

/styles

  /components
  
    my-component.css

and in the markup

<div class=".MyComponent-wrap">

  <div class=".MyComponent-title">  
    ...  

  </div>  

</div>

With CSS like that you always know where a style is declared, and the order of the classes do not matter.

More on the issue: http://nicolasgallagher.com/about-html-semantics-front-end-a...