It could be a great problem if users don’t understand that they can click on a button, even worse, they might not notice that that button is actually a button. Then you have users moving the mouse around the website, trying to find something to click.

To avoid this usability flaw, try to include 3D buttons in your interface design. Most users understand the message because it represents their mental model and the real life: oh, it can be pressed!

In the example below, taken from Google Calendar, the arrow buttons are totally flat and it’s difficult to notice that they are activated.

Expedia.com, with its “Search for flights” button and Lego, with its “Search” button, give users a clear message: this is a button, is activated and you can click on it.