I understand this is opinionated and "personal", but on the off chance that this might make React less intimidating, let me chime in and say that you can do professional work (and build pretty much any reasonable application) without:
- really understanding "semantic HTML" (if you know what div's and span's are, you're ready to get to work),
- understanding flexbox (you can, and many pro developers do, just use a CSS framework, and they're especially nice in React because they're delivered as components, so all you really need is the documentation for the layout components of the framework you pick),
- design patterns --- in fact, I might argue that you're better off not reading about design patterns, which are kind of an architectural blind alley,
- CSS preprocessors, which are neat but inessential,
- CSS architecture,
- Redux --- more nascent React developers have been derailed by the suggestion that they need to grok Redux than by anything else in the ecosystem; you don't even need to know what Redux is to build perfectly good React applications,
- async actions, which are another aspect of Flux, which again you don't need to grok,
- react-router, which is a disaster of an API that abstracts something you can build in just a hundred lines of code or so, and can in fact do without altogether,
- lodash and moment (this is nitpicking, since neither of these demand more than a couple of minutes to pick up, but just to be clear: you don't need them, especially in ES6).
- really understanding "semantic HTML" (if you know what div's and span's are, you're ready to get to work),
- understanding flexbox (you can, and many pro developers do, just use a CSS framework, and they're especially nice in React because they're delivered as components, so all you really need is the documentation for the layout components of the framework you pick),
- design patterns --- in fact, I might argue that you're better off not reading about design patterns, which are kind of an architectural blind alley,
- CSS preprocessors, which are neat but inessential,
- CSS architecture,
- Redux --- more nascent React developers have been derailed by the suggestion that they need to grok Redux than by anything else in the ecosystem; you don't even need to know what Redux is to build perfectly good React applications,
- async actions, which are another aspect of Flux, which again you don't need to grok,
- react-router, which is a disaster of an API that abstracts something you can build in just a hundred lines of code or so, and can in fact do without altogether,
- lodash and moment (this is nitpicking, since neither of these demand more than a couple of minutes to pick up, but just to be clear: you don't need them, especially in ES6).