Meaningful icons are very important for a website usability. Without proper signs users will not know how to use your site. This sounds obvious but it is not, there are many bad examples.
DZone is a digg-like service for developers, I like the idea. Unfortunately their website have many usability problems. The one that drives my attention today is the excessive use of icons that don’t have a so obvious meaning.

In the picture above, you can see the option icons DZone use for the “Spy” section. Unfortunately, and this is very bad, as users hover over the icons no label is displayed.
Keep in mind that usually what is obvious for you is not for your users.