Here is a checklist of knowledge and abilities which I consider a must for a good web programmer.

• HTML, w3.org standards and validator
• AJAX
• SQL: multiple column indexes, transactions
• The following programming languages exist: PHP, Python, Ruby.
• Program in at least one of the languages above
• Program in C at least a bit
• What is JVM and bytecode
• What is SQL injection and how to prevent it
• How DNS works, at least in general – what server and clients do
• How routing works, at least in general. Tunneling. NAT. BGP is a plus
• HTTP and HTTPS
• Load balancing and it’s session persistence problem and how it’s usually solved
• HTTP proxy, what it solves and how it works
• What’s FTP
• What’s version control and what it solves
• How SMTP works, at least in general. POP3 and IMAP a plus
• What’s character encoding and why UTF-8 rules

If it’s someone expected to work with Linux the following applies:

• SSH
• Shell scripting: bash, grep, sed, awk, find, netstat …
• File-system permissions

I’ll be adding to the list as soon as I remember anything important enough.
Suggestions for additional points are welcome.