Usability is extremely important, and yet there are so many products that violate a lot of rules. There are several questions on StackOverflow that are about usability (see: link1, link2, link3), however what I feel is missing still is a comprehensive list of usability "tactics": concrete examples of what (not) to do for a web application.
Please don't add references to books. Please list one example per answer so that we can use the voting to actually prioritize the list.
Show, Don't Tell
Whenever possible, meaningful visual cues (when appropriate to the audience) should be chosen instead of lengthy textual descriptions.
Use affordances to make controls understandable. Avoid confusion between emblems, banners, and buttons.
Make good use of whitespace. It helps to make things much more clear than just having everything crammed in competing for attention.
Use a statistics package to monitor traffic through your site.
Which pages pique user interest? Which pages make users leave? Adjust your site accordingly.
Don't reinvent the wheel - use tried an tested user models so the user elements behave as the user expects.
I wish more sites would make better use of tooltips (i.e., the TITLE tag on links, images etc.)
In these days of AJAX applications it's not always clear what is going to happen when you click on a link - a brief description would help!
Don't hide functionality in a dropdown (users can not explore the possibilities your application offers)
Use Standard Controls: Don't create new controls unless there is a clear reason to do so. Users are used to the standard controls and know how to use them.
If a button does one thing on one screen, and the same button appears on another screen, it better perform the same action as it did on the previous screen.
Be Consistent: There should be a convention accross all pages / dialogs / screens that maintains the users expectations when using a control or performing an action. This could be the caption on buttons, the position of elements or the steps required to perform certain actions.
Do REST-like links, so user can easilly bookmark or copy-paste your site. This is often forgotten in todays AJAX-times.
I've always been partial to the Principle of Least Astonishment, which stipulates that the best user interface is the one that minimizes any surprises to the user.
In other words, make it do what the user expects it to do!
Corollary: Users don't read manuals. Or help text. Heck, users don't read!
Study up on User Stories, and treat them like a Holy Grail. Then get user feedback to improve them, and to learn what you can change. Pay special attention to what you can remove. Then pay next attention to what you can simplify. Consider all options before adding anything.
PS - We are all, to a certain degree, UX "experts". UX is not just about UI in the computer world. On the contrary, UX, aka user experience, applies to every thing and any thing we interact with in the real world.
Example of good UX: - Simple coffe machines - Inserting a VCR cassette - Pencial sharpeners - Q tips
Examples of arguably bad UX: - Complex, computerized coffee machines from hell - Programming VCR's - Motorized pencil sharpeners that frequently jam - Cheap Q-tips that break or fall apart in your ear