I've been I've asked around, and here's what I gathered so far in no particular order:
Knowledge
Web server management (IIS, Apache, etc.)
Shell scripting
Security (E.g. ethical hacking knowledge?)
Regular Expression
HTML and CSS
HTTP
Web programming language (PHP, Ruby, etc.)
SQL (command based, not GUI, since most server environment uses terminal only)
Javascript and library (jQuery)
Versioning (SVN, Git)
Unit and functional test
Tools
Build tools (Ant, NAnt, Maven)
Debugging tools (Firebug, Fiddler)
Mastering the above makes you a good web developer. Any comments?