I begin with the assumption that people get on the internet to get information about a product or service - not to look at pretty pictures.

Test the site on a dial-up connection.Picture of my "Pride & Joy"!
A properly designed page will display in less than 8 seconds - if it's longer most visitors will move on to another site. There should be readable text almost instantly. Give me something to read and I will stay while the graphics load (if it does not take too long!). There should be very little text in graphics and then only for things like a logo. The page should not jump around as the graphics are loaded. The pictures should be as small a file size as possible to speed loading. Do you want to wait 120 seconds for a picture of my "pride and joy" to load? Don't believe for a minute that everyone with optional income is on a high-speed connection.
Mouse over the pictures on the page.
For usability (visually impaired and/or people surfing with graphics turned off) a pop-up tool tip should display what information is missed by not seeing the picture. This also increases the word count on the page without overcrowding. One of the items used to determine how near the top of the search your site is how many times the keyword appears. Even though I am vision impaired (or my office has graphics turned off) - you still want my money, right?
Test the site at several window sizes.
If you grab the window at a corner and change the size, how far can you go before a horizontal scroll bar or too much "white-space" appears? Screens are getting larger every day but not all people use their browser at maximum view size. Statistics showing that most visitors are at 1024x768 are at best misleading. Additionally some surf with history, favorites or search using up part of the view port. The site has to be able to be viewed by cell phones etc. A flexible design is a must.
Test using a minimum of three browsers.
Use at least Internet Explorer (IE), Mozilla (Firefox/Netscape), and Opera. I recommend testing in that order. IE may not be the best browser. It is the one most used - up to 95% in some markets. To download these and other browsers search Google using "download browser".
The page should look good at all text sizes.
Using IE change the text size by selecting View, Text Size, Smallest to Largest. Improperly designed pages will not resize in IE. Other browsers can resize this text - Opera even resizes the images. Resizing should not cause text to overflow other blocks of text and/or graphics in any browser. In all honesty the most important text size is medium for IE - 100% for the others. There is no valid reason for the page to "break" with larger or smaller text however. Tip: Pressing the [Ctrl] key while rolling a mouse wheel will also change text sizes with "most" modern browsers.
Ease of navigation.
Links must look like links. They have to stand out as clearly different from normal text. Some sites have links that can only be found by mouse over. Irritating at best.
Design elements put in to show-off.
The designer learned a new trick, fell in love with it and put it on the page. I fell into this trap - I think most designers have at one time of another. Splash pages come under this category. The splash page is a very large image or flash movie containing no relevant information but inviting me to enter their site - I thought I just did!
Counters
Inaccurate, amateurish looking, and of no interest to the user. Some web design software even allows the designer to set the counter to a high number to impress somebody. Much more complete information is available behind the scenes without bothering the user or adding to the paranoia about the Internet (privacy et. al.).
Frames don't show your site at its best.
There will be scroll bars (horizontal and/or vertical) out in the body of the page. They make the page hard for search engines to index. If the search engine does find/display the page, it will be without the frame. The frame has all your navigation, logo, and other base graphics - usually including your contact information.
Feel free to print this page.
This page is designed to print properly on most browsers. The navigation and other silliness will not print. Critiques and thanks are gratefully accepted.

The main goal of this site is to help (in however small way) reduce the number of really bad sites.