1: Thinking your reader loves you
They don't. They are there to find information quickly and easily and then they will move on. Give them bite-size pieces of information to draw them in and always remember you are writing for the audience not for yourself.
2: Padding out the content
Website writing is not about quantity: it's about quality. Write short, succinct paragraphs and use bullet points where necessary to draw out key information.
3: Assuming the reader will work it out
No they won't. If you can't spell, have bad grammar, or can't structure a sentence, stop writing! Find somebody else to do it for you.
4: Thinking people read your site like a book.
Writing for the web is not the same as writing for print. Web writers write for how people use the web, not for how they read. Don't assume any reader has seen any other page on the site. They probably haven't. Treat each page as standalone content which links to other content, but doesn't repeat it.
Repetition drives users away, whereas links are what the web is all about. So link, don't repeat.
5: Writing clever headlines.
Headlines are the single most important piece of content you will write on the web. Headlines get the attention of readers, make it clear what's going on, and (most importantly) are very attractive to search engines. Keep headlines short (ideally no more than eight words). Keep them clear and descriptive. Avoid being "clever" or using puns and wordplay unless there's a very strong communications reason for it. Remember, Google has no sense of humour, and won't appreciate a witty headline. It just wants the facts.
6: Underestimating the value of good content.
Content is king. It's as simple as that. Good content builds your brand and your reputation.