Well I think that the first place to discover how to develop in Genesis is the StudioPress platform itself. It contains a lot of information on how to move your first steps with this framework. You have to know the Hook system of WordPress because Genesis it uses them extensively and is able to optimize lot of its codebase because of it. Another WordPress important knowledge to have is the child theme logic that you have to know before you can call yourself a Genesis developer. If you need some video tuts you can check the new course in Lynda.com of Carrie Dills based on How to create a Genesis Child Theme where you can understand better the use of hooks and child themes. After that you have to start and create some websites and learn how this framework plays with other solutions. Hope this was helpful and don't hesitate to ask follow up questions ;)

Guys I read all your answers but I think there is one plugin that you've missed; and I am talking about the Jetpack . This is an amazing well made plugin made from Automattic and from the others it gives you a CDN just for images and it will resize them as well , I advice you to read their API because it gives you so many ideas ;) Beside that since the 4.4 version you souldn't worry anymore even of the pixels dimension of your images because from now on WordPress supports the srcset attribute to serve different sizes on different devices. Hope this gives you a little overview of Jetpack because there are plenty of useful feature inside this single plugin ;)

Hi Matthew, it definetly can! The missing part is the lack of a drag'n drop builder but the config is organized with arrays and is really simple get accustomed to it ;)

Well Underscores is surely one of the best starter theme out there and the fact that has been developed and maintained by a bunch of really great WordPress developers (some of them works right at Automattic) let us imagine that the code is 100% WordPress. At the same time I have started to not use a starter theme anymore even if at the beginning I was using them a lot ( Bones is a great one too) because all of them require me to build my on starter elements like the grid system and several other feature. Nowadays me and my there have chosen to use the Genesis framework to help us jump start the development phase and we choose the Genesis framework because, even if it is a commercial one, we are able to start with a solid codebase and with the help of child themes and common coding standard we're able to deliver the project in less time. I understand that my answer is a bit different from your kind of request but at the same time it has been my path into becoming a better WordPress developer :D

I think that Flexbox is really easy to understand, you can try and play with it on http://flexboxfroggy.com/ it will help you understand how the single property play together with real-time feedback and lots of fun! Correction: if I do not get wrong there will no be CSS4 since the development on this project has been setted a per-module feature, like HTML5, so it is going to be pointless to add a version number. We see how this will play out since will be really difficult to understand how we can talk to each other :D

I prefer a lot CMB2 since the code knowledge required is really little and the project is open source and a LOT of developer are expanding it writing modules and new plugins. I tried ACF few times and even if the UI is really helpful to create Custom Meta as a developer I always prefer to include the CMB2 library inside my project and work with it without the need to install a new plugin.