In general, "good" sites:

• Load quickly (5 to 30sec)
• Have good content presented in a concise and well organized fashion
• Have clear navigation that is easy to understand and use
• Have an attractive "look" to the layout and effective use of graphic elements (contrast, repetition, alignment, proximity)
• Designed to have the elements that improve the likelihood it will be indexed by search engines
• Are interactive, encouraging the user to send an email, fill out a form, request information, etc.
• Function properly on various platforms, browsers and computers for the target audience
• Have current information and have new/changing things on each visit
• Have complete contact information (email, phone, fax, address)
• Have an appeal that will bring a visitor back to the site

On the other hand, "bad" sites:

• Load slowly with out giving the user the option to choose to down load large files
• Are artistically lacking (poor color combinations, inconsistent themes, too busy background texture, poor image quality)
• Are hard to figure out (inconsistent format, unorganized, confusing navigation)
• Have too many bells and whistles (use of not fully supported technologies, distracting use of animated gifs, endless looped sounds, moving text)
• Require you to scroll to view (poor planning in considering the end users machine limitations)
• Contain old or limited information

To get a sense of some of the things to avoid, visit http://www.zdnet.com/yil/content/growth/
computer/oopsie.html that links you to sites that take absurdly critical looks at current web sites thrown out there.

What makes a good web site?
This is a subjective question though as with any art, there are good guidelines to consider. A well-designed site requires the balance of good content, artistic use of graphics and a knowledgeable application of web technologies. It's wonderful that anyone can build a web site yet few can pull it all together into an effective site. The column on the left is a bullet point list of some "good" and "bad" characteristics of web sites.