I think a good website is one that targets the audience well, one that creates a strong brand and identity. Of course, a decent website is also one that has the fundamental elements of webdesign in practise (accessibility, functional, easy to navigate, fast loading etc) combined with an attractive and eye pleasing design.
1. Content that delivers useful information to the reader so he wants to read on.
2. Well written so the user can understand what is being said.
3. Navigability so the user can find what he wants
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I remember reading some article about this issue. If you're talking about a commercial web site, there are lots of things involved.
Some parts I can remember from that article:
* Try to make it compatible with different browsers, operating systems. (providing HTML-only, TEXT-only versions along with dynamic ones)
* Try limit of the text in single page (cannot remember the value).
* Try to organize your links carefully so your customers / visitors can go to desired page in not more than 3 clicks (according to a research, most people will give up and visit somewhere else if they cannot make it in 3 clicks).
* Using of colors, font sizes, font types.