This one is very short, but I think these tips are essential and basic and unfortunately I see many many sites doing exactly the opposite and it is a pity:
- Don’t use flash.
- Don’t have some content as HTML and some as PDF or documents in other formats. You can transfer the content in HTML.
- Don’t have music playing. Please don’t.
- Don’t have animated banners or other moving parts. They distract the reader.
- Use a bigger font and watch spacing.
- Don’t have scrolling text. Please don’t.
There are many many more pitfalls, but I’ll leave you with these. If everyone used these tips, we would have a much better web presence.