320 and Up owes a few details to Bootstrap, as it uses the latter's styles for buttons, forms, and tables. In fact, the raw design resembles an unstyled version of Bootstrap, but there are enough new features here to set 320 apart. For example, 320 includes CSS3 media queries for five basic increments of device screen size, so you can build hints into your design to work with those sizes. The framework also bundles LESS and SASS for programmable control over CSS and Modernizr and Selectivizr for backward compatibility with the dreaded IE6 through IE8. The creator also packed in a responsive typeface test suite and design test page to make the job of checking your work a little easier.
My favorite feature is Font Awesome, a font-based icon set that can scale to any size, originally developed for Bootstrap. Granted, you can add it to most any framework on your own, but having it bundled is a nice bonus.
320 and Up uses Bootstrap for many of its object styles. The difference? 320 has been built for designing mobile-first sites that scale up, instead of desktop sites that scale down.
52framework52framework bills itself as "the framework from the future," which in this case means support for HTML5, CSS3, and all the cutting-edge features associated with those things. 52framework is most useful for getting a leg up with advanced HTML5 features, especially the likes of video and local storage, and for having frameworks for handling form inputs. It's less effective for building a design that's both mobile- and desktop-friendly, since other frameworks here have more pieces for those tasks.
Most of the pieces inside 52framework should be familiar: It uses Modernizr and Selectivizr for backward compatibility, sports a grid-layout system, and provides a broad range of CSS3 property controls. What I found most lacking in 52framework was the documentation. Instead of textual documentation, as with Bootstrap and Boilerplate, you'll find a series of YouTube-hosted video demonstrations. They're useful as adjuncts to existing documentation, but not a substitute for it.