You may not realize it, but a lot of web developers still only know things like ASP, .NET, ColdFusion, PHP, etc.. So knowing modern languages and APIs like Go,

You may not realize it, but a lot of web developers still only know things like ASP, .NET, ColdFusion, PHP, etc.. So knowing modern languages and APIs like Go, Ruby on Rails, Python, NodeJS JavaScript, etc. is already a step toward specialization. Another frequent ask I see from clients is knowing modern streaming methods like Web Sockets, Comet, Meteor, etc. which is another level again. Past that their is mobile development, which can be done in HTML/JS on platforms like Cordova, and also scaling and deployments. It's often different to write something that can run on hundreds of servers at once than a single web server, for example.