Just 10 years ago, you could be brilliant web designer if you just knew how to set up a simple table, with a navigation bar and a space for content. Now, it's a lot different. You now have to consider better colors, balance, shape, and pattern.
Here are some common mistake made by beginning web designers in today's web design era:
1) Balance
Our mind is very aware of how a web page balances out, and it affects our attitude about a web page more than you may think. A centered layout is always a simple solution, but you can be more original than that. If the navigation column has a darker background, it should be smaller. In our minds, dark is heavier than light, and wide is heavier than thin. Having a thin, dark navigation bar and a wide, lighter content area, will balance the page out. Thats a simple task, however. Keep this general rule in mind when adding other elements such as headers, footers, news boxes, etc.
2) Navigation
I'm going to leave your website right away if I can't find what I want. Have a separate area for general navigation. A common mistake I see all the time is no navigation area, but a bunch of hyperlinks mixed in throughout the content. Don't hide your navigation.
Along with not knowing what to do with your navigation, some designers try to do too much. They try to come up with the most original and interesting way to display their navigation, often hiding them within image maps or displaying alternative code words for common pages.
3) Text Size & Font
In the world of online reading, Times New Roman, or any other typing print is bad. These fonts are easier to read on paper, but on the screen they are more difficult. Try a cleaner font like Arial, Verdana, or Tahoma.
Also, don't get carried away with text size. I've seen text too small, and too big. 9-11pt font is a fine size for content and navigation. Header texts should be bigger, but don't get carried away. You want your text to flow when a visitor is reading it-they should not have to decipher.
4) Color
Colors should match, but still vary. It adds a lot of style to a website to have two or more colors, but you need to be sure they look good together. If your still uncomfortable mixing colors, stick to one. To be on the safe side, a good website resource for color palettes is Colourlovers.com. Bright colors are sometimes a problem-you don't want to blind your visitor! Be careful with these colors by balancing them out with neutral colors of grey, black, or white.
5) Header Size
If your going to have a header image on your website, keep it's size less than 1/3 the size of the whole page. I've seen some headers where I had to scroll down an entire page length to see any sort of content. Its distracting, and doesn't complement the balance rule. Also, keep it fairly simple. Have one main theme, not a collage of a thousand themes. You want your header to compliment your website, but you don't want your website to be an accessory of your header.
Keep these general rules in mind, but also keep learning new techniques to improve your skill.