What should I know about building a website?

This is a reprint of an excellent collection of answers given by users at http://programmers.stackexchange.com. Original post is here.

The idea here is that most of us should already know most of what is on this list. But there just might be one or two items you haven't really looked into before, don't fully understand, or maybe never even heard of.

Interface and User Experience

Security

Performance

SEO (Search Engine Optimization)

Technology

Bug fixing