We also focused on things for highly advanced Sass users to better use our mixins. The grid mixins are really powerful and can be used to quickly create layouts of all grid sizes.

They came from a similar beginning, so have a lot of similarities, but a lot of philosophical differences. With this version of Foundation our goal was to make it as slim as it can be without removing the functionality people need to get going fast. We removed many of the presentational classes that seem frivolous like round, classes for buttons or spacing classes for items. These are convenient in the moment, but can clutter a code base really quickly.

Yeah, we totally understand the upgrade concern. The short answer about upgrading is you really don't have to, since Foundation isn't like wordpress or Drupal where there are security implications. ZURB has about 30 properties around the web and some are running on Foundation 5, 3, 2 and even pre-releases on 1. If we think the property will benefit we'll upgrade we will if not we won't. The main benefits here are to file size, customizability and navigation patterns. We're doing a client upgrade now because they wanted to benefit from some of the additional Sass grid tools and then eventually be in a place where they could swap to the Flex grid and shrink file size even more. Overall we'll upgrade some sites, but not all. Hope that helps :)