What are the most Important Usability, Accessebility and SEO guidelines??

Can anyone tell me some of the most important usability, accessibility and SEO guidelines, which must be taken into considerations while developing a good Web2.0 website ???

From stackoverflow

• http://www.w3.org/WAI/
  http://www.section508.gov/index.cfm?FuseAction=content&ID=12#Web
  http://www.google.com/support/webmasters/bin/topic.py?topic=8522

  Don't get too hung up on the "Web 2.0" buzzword. Accessibility standards have been around for a long time and are an important part of any web site.

• SEO hint: Rewrite your dynamic URLs so that even the actual URL may be dynamic, it appears static. This allows spiders to fully index your site.

  On Apache, you can do this by using mod_rewrite and there are symilar techniques for IIS.

  Example:

  Instead of

  http://yourdomain.com/cgi-bin/getproduct.pl?id=12345

  use something like

  http://yourdomain.com/products/12345

  As an alternative (or additional measure for those dynamic URLs you cannot rewrite), you should create a site map.

• Incidentally, there was a recent article on it.

  Other good Usability articles:

  • 30 Usability Issues To Be Aware Of
  • 12 Useful Techniques for Good UI Design in Web Applications
  • 10 Useful Web Application Interface Techniques/
  • 10 Usability Nightmares You should be Aware of

  While I dislike the 'horror' tone of the article headlines, the articles themselves are good practice.