Pointers for 2015 Web Application Concepts/Methodologies- MVC, HTTP, etc

I am building an app for a SV startup in the near future and would appreciate some pointers in terms of concepts to focus on.

I already know about RWD, DOM, HTTP GET/POST, JSON/AJAX, MVC Server Side Architecture, and Relational Database Design. I am currently learning RESTAPI and NoSQL Database Design.

Other than what I have listed, what concepts/methodologies would you say are imperative to building a secure well performing web app that I should look into before picking out the toolset (framework language stack and automation tools)?

Greatly appreciate the inputs.

Thanks