Here's what a few folks have to say on the topic.
I’ll argue that front-end developers need to master four different skills.
Front-End Developers, having learnt HTML, CSS and JS, are forced to take functionality into account when creating user experiences or making sure that the two disciplines work as one from a development perspective. They become all-rounders, having to understand what is actually happening between the AJAX data and the PHP file that’s sending off a mail or returning errors.
... read the whole story at css-tricks.com.
In my opinion, what defines a good front-end developer is one that has skilled knowledge of HTML, CSS, JS with a vast understanding of User design thinking as they'll be building web interfaces with accessibility in mind. They should also be excited to learn, as the world of Front-End Development keeps evolving. The ability to stay in the loop is critical.