There are plenty of mistakes in the world of web design. Let's look at what I believe to be the 10 biggest.

1. Too Many Ads.
When you're trying to make money from your website, it's all too easy to overwhelm your site with ads. Put yourself in your user's place and take a good hard look at your site and ask yourself if the ads feel intrusive. Does the site look like an information source or does it feel more like a page-holder for the ads?

2. Plugin Overload.
You have to keep plugin useage to a maximum of 1 type per page. If you've got Flash, then you can't have a media player, or if you're using Java, then no Flash. It's not as bad to use the same plugin twice, however.

3. Flash Intros.
Please, don't use a Flash intro on your website. They have been so overused that they're becoming universally mocked.

4. Unclear Layout and Navigation.
Many websites, especially business sites, suffer from some kind of disease where the very simplest task takes 10 steps. If users are asking you how to do things on your site, then you need to improve your layout and navigation. If there are certain tasks people want to do frequently, put them on the front page.

5. No Marking for External Links.
There are 2 kinds of links: internal (to other parts of your website) and external (to other websites). For the benefit of your users, it's best if you mark external links, either by making them a different color or using some kind of a symbol (a box with an arrow is the usual one). It's also good to make the external links open in new windows, so people aren't leaving your site altogether when they click them.

6. Unclear Linking.
Some web pages are designed to show links only when people put their mouse over them. While this might make the design look nicer, it is not very user friendly. Instead, use a clearly contrasting color for links, and preferably underline them. This makes them more visible to the user, thus more user friendly.

7. Unlabelled Email Links.
Always clearly mark a link that will send email (a mailto link) with the word 'email'. If you turn clicking a name into send email, you'll annoy users who expected to find out more about the person.

8. Broken Links.
You should check your links regularly to make sure that they all still work. There's nothing worse than finding a site that looks useful, only to find that it hasn't been updated in years, so most of the links don't work. While a website does mostly run itself after a while, that doesn't mean that you should neglect essential maintenance.

9. Strange Fonts.
Stick to the most common web fonts: that's pretty much just Arial, Georgia, Tahoma and Verdana. If you use more obscure fonts, then most users won't have them -- and those that do will find your text hard to read. The only place for non-standard fonts is in your logo or headings, and then only if they are displayed as an image.

10. Badly-sized Text.
It's important to keep your text around the standard size. Making text too big or too small makes it hard to read and annoying for many users. The best thing you can do is use relative text sizing (not pixels) that allows the browser to respect the user's preferred text size. You should also consider offering buttons on your site to decrease or increase the font size.