Here are a few specific things that either I look for or I have been asked about:

You should know how the web works -- be able to explain, for instance, how cookies work, or what happens from the moment you click a link to the moment a web page appears in front of you. Don't worry about anything below TCP/IP here.

Also, be competent at relational databases -- if someone gives you a problem, be able to correctly design a database for it, quickly. Know what normalization is and when to use it and when not to.

Have read a book on design patterns.

Be able to write code on a whiteboard.

If you list a programming language or technology on your resume, be prepared to answer questions about it.