Web of Horrors
 
There are many Web designs that actually work to scare people away from their sites, rather than keep people coming back. These designs should be avoided if you don't want the site to be labeled the "Worst of the Web".

Backgrounds

There are so many things that the designer can do with a background on the Web site, but many of these tricks are ugly, hard to read, and just generally scary.
Black background with light, white, or pale text
Unless you're a professional designer, you should avoid this design technique. Even when it's done well, it can look pretentious, and it's VERY difficult to do well. Plus, some browsers try to print the background color - resulting in angry customers when their new ink cartridge is used up printing the page
 
Black with dark text
 
It's illegible. All the objections to black backgrounds still apply, but now you've added in the bonus that your customers can't read the text unless they highlight it all first. This is annoying.
 
Tiled background images that are too busy
 
This is very popular with new Web page designers. They learn both how to add tiled backgrounds, and so decide to tile a picture of their cat that is 40x60. Or they just pick a background tile that is plaid or otherwise illegible. Tiled background images add to the download time, and they almost never add recognizable value from the customer's perspective. The one exception is a wide tile (wider than your screen) to create a left-side gutter on the screen
 

Text

Text is what people come to the Web page for. If they can't read it they'll leave. And these text techniques guarantee that they will leave.
Everything centered

This is one of the most popular design tricks for new Web designers. Possibly because it makes it obvious that you've actually affected the design of the site. But it makes the page really hard to read. And it's often very boring looking anyway. There is nothing inherently wrong with centered text. As with ragged right or fully-justified text alignment, what works for one design might be totally inappropriate for another layout. There are simply fewer situations where centered text is appropriate. When in doubt, don't center it.
As with all layouts, alignment depends on the purpose of the piece, the audience and its expectations, the fonts, the margins and white space, and other elements on the page. The most appropriate choice is the alignment that works for that particular design.
Too many headlines, emphasis, strong, bold, or italics

Don't have every line on the page be a headline. This gets really boring looking and the readers no longer know what is really important.

Blinking text

Blinking text is nearly impossible to read. And if you do something like surround an entire paragraph with blink tags, it's like a strobe light. People don't try to read it, and unlike a strobe light, you can't even dance to it.