A few things worth a mention. Most of these design practices should be avoided at all costs. A few are tolerable if used in moderation. This page is based on personal attitude and research.
Things to avoid - big bads
Not responding to emails - This is the fastest way to loose a visitor/customer.
RESPOND TO YOUR SITE EMAIL WITHIN 24 HOURS!
Slow pages - not everyone has a fast ISP connection or a blazing CPU
Long Pages - that go on and on and ... and I got lost
Broken Links - missing internal site pages or dead external site links
Updates - never changing or adding new info and content
Music, Plugins, Flash intros - can annoy people and may cause them to leave
Popup or Popunder windows - this is just a plain rude and obnoxious attribute
Too many ads - if your site is for a newspaper flier, go ahead
Major Typos - spell check and have a few people proofread it
Little/No text - 20 pics and 1 paragraph is fine for an archive or a photo album
Acceptable use notes
Moderately Longish Pages are only okay if the visitor knows and is prepared. If you can make a long page why not split it up into several? Then give the visitor the choice to click the link or not.
Music, Plugins, Flash intros, and such are typically very annoying. If they serve a function or pertain to the website, give your visitors a choice to use them or not. If they start up when the page is loaded, and somewhere on the page is the off button, that is not a choice.