This Meet Magento New York 2014 talk is Brendan Falkowski's "Two Approaches to Responsive Design" presentation. Adaptive vs responsive - adaptive design serves a specific size; you basically have 2 sizes - small screen and large screen. It doesn't support both tablet and mobile well.
Adaptive layouts don't work well for new or 'in between' screen sizes - i.e., the iPhone 6+ screen size isn't served well by adaptive layouts.
Responsive proxy sites are a good middle ground - build a mobile-first responsive design on an m-dot site, keeping a separate desktop site until you're able to change your desktop site.
Dynamic serving involves looking at the user agent and other data and dynamically serving only the items needed for the user's screen size and device type.
Responsive design with server-side components blends responsive design with dynamic serving.
Badly written or blocking JS/CSS can derail search spiders.
Some RESS implementations use MobileDetect or WURFL frameworks to implement RESS.
RESS's biggest impact on UX was navigation - being able to serve completely different navigation menus to different devices allows for better usability and easier design.
Magento responsive sites can be and are fast - focus on the things that are important on the frontend and your site can be fast.
Responsive has a lower total cost of ownership since you're only maintaining one codebase.
Retina images are tricky - the best approach for now is a plugin called picturefill. https://github.com/scottjehl/picturefill #mm14ny