Consider the websites you frequent most and how you use them. Do you ever get frustrated? Have problems finding what you're looking for? If yes, than you are already familiar with what is probably the most important principle of web design: Usability.
What makes a site usable? What doesn't? This isn't an end-all be-all list of web design snafus and successes, but here are five dos and don'ts that should give some basic guidelines to good web design.
DON'T hide the log-in from your site visitors so they have to scroll and search for it.
DO make the log-in more visible by increasing the font size or setting it apart visually in another way.
DON'T use pop-ups to present content, your users will be too lazy to disable their pop-up blocker.
DO respect your visitors and the time they spend on your page by not interrupting it.
DON'T use a font size that is too small or a color that is too bright to be legible.
DO choose a simple, no-frills font (like arial) and be consistent with it throughout your site.
DON'T have more than one flash animation per page.
DO make sure the focus of each page is on the content.
DON'T overwrite or use obvious marketing language in web content - it will be skipped over.
DO use concise language and bullet lists.
Want to know more? Check out this article that outlines 10 Principles of Effective Web Design.