What are the key differences in writing for the internet / intranet?

When writing for the Internet, you are writing to a much larger population from a more diverse background(social/educational/economic and political etc..) so you have to taylor your style to meet broader needs without pissing anyone off. The Intranet is likely a much smaller group of readers who share similar interests and backgrounds, usually within a company. So, you can make umptions about the knowledge level of the group of readers for which you are writing.