You can also use whatever off-the-shelf image rotators, social medial share buttons, or video plugin that you choose.
For responsive design projects, we've been using Unsemantic and Normalize. Unsemantic provides a great framework for laying out on mobile and tablets, and Normalize is just a good CSS reset.
The CSS files are brought in via a standard .NET theme.
Once that is done, you use Unsemantic as you would on any other site, by properly associating classes to divs.
We don't use the Layout Manager in Sitefinity much, as it can add too many divs to your HTML and doesn't provide the granular control that we need for our HTML.
We also don't use the Responsive Design Module, as the tool (to date) makes many global configurations. Such as, all three column divs will look like a single column on mobile. You can't say that a three column layout on one page looks like a single column and on another page it will look like two columns and a large single column underneath. All of the rules are all or none.