I think this can be answered from the user’s perspective and from the developer’s perspective. I think a page can be considered well designed if it looks good, works with many browsers, and can be maintained by others than the original author. From the user’s perspective, I was able to come up with the following list:
Accessibility - the page is compatible with screen readers and alternate input devices. At work we recently went through a remediation process with one of our web sites. We needed to assure HR the site was compatible with accessibility utilities. I think about 75% of this can be handled by writing good HTML source. In addition to this, testing tools such as WebKing can help identify other problems that can prevent the web code from working in certain situations.
Navigation - the page is easy to leave. Another way to say it is the page should have the necessary links to navigate away to other major areas, if it is part of a larger web site.
Placement - the page is easy to find in the site and navigate to.
Compatibility - the page can be loaded and properly displayed in popular browsers. I think in e-commerce, it is important to give this item some amount of priority. You want to encourage visitors to browse and buy regardless of the specific brand or version of their technical resources. This is also important to consider if your viewer base consists of users with handhelds or Internet-capable cell phones.
Organization - information on the page is presented in a visually appealing way, including text style choice and page positioning.
From the developer’s perspective:
Documentation - comments in the code or a short design note helps the author remember what they did and helps other maintain the page later.
Organization - the page’s source is consistently organized and formatted into blocks. I think with today’s tools that can reformat source code, this is less of a problem.