While very important for mainstream markets, it's not something that blockchain developers need necessarily to focus on.
I disagree a bit, as I remember from the time when BTS 2.0 was coming into existence that blockchain data needs to be properly exposed in the form of a robust and comprehensive API - otherwise creating a responsive GUI is much harder or even impossible. I remember Dan talking about it at that time and him even coding a bit in JavaScript in order to better understand the front-end perspective.
Good point about the API. It's so important that it might be reasonable to add it to the list.