1. Site Load-time Is Reasonable
Call me old-school, but I still like to see sites come in under 100KB (60KB is even better). If a site takes forever to load, most people will just leave. Yes, many of us have broadband now, but that makes our patience even thinner.
2. Adequate Text-to-Background Contrast
Dark-gray on light-gray may seem stylish, but I'm not going to ruin my eyesight to read your blog. Eyes and monitors vary wildly, so keep your core copy contrast high. Good, old-fashioned black-on-white is still best most of the time.
3. Font Size/Spacing Is Easy to Read
Opinions vary on the ideal size for text, but err on the side of slightly too big. Poor readability increases frustration, and frustration leads to site abandonment. Also, make sure your line spacing is adequate - white-space is a designer's best friend.
4. Flash & Add-ons Are Used Sparingly
No matter how great your site looks, people won't wait 5 minutes for a plug-in to load. Use new technology sparingly and only when it really enhances your goals. Sticking to standard HTML/CSS is also a plus for search engines.
5. Images Have Appropriate ALT Tags
Not only do sight-impaired visitors use ALT tags, but search engines need them to understand your images. This is especially critical when you use images for key content, such as menu items.
6. Site Has Custom Not-found/404 Page
If a page on your site doesn't exist, a white page with "404 Not Found" is a good way to lose a customer.