Web design encompasses many different skills and disciplines in the production and maintenance of websites. In my opinion, these are the most important principles in web design, arranged by importance:
I would say that the first principle is loading time. No one will enjoy your design if it takes ages to load. The longer a page will take to load, the more users will quit your website for another one.
The second most important principle is responsive design. It means that the design of a website adapts itself according to the user's screen resolution. No one likes to visit websites designed for 21 inches screens on a 4 inches smartphone screen. Everyone should be able to browse your website, no matter the resolution of their screen.
The third principle is purpose. If the users don't understand in a few seconds what your website is about, then the design should be reviewed.
The fourth principle is navigation (links, menus). The easier it is for a user to find its way on your website, the more (s)he will keep browsing it.
The fifth most important principle is visual identity (logos, typefaces, colors). It's what makes your website graphically different from the concurrence and easily recognizable.
Web design is all about satisfying the user's needs. If they don't understand what they're looking at, or if they struggle to find information or to load the page, they will leave for another website.