It's a common, and unwelcome, comment during consumer website usability testing. Users first ignore the page's most prominent feature, then fumble around the page's links and text for awhile, before finding the feature some other way or stopping in frustration.
When you ask them if you saw the big, carefully designed component for Feature X, they respond to show you that they perceived it, but they didn't really see it. They thought it was an ad.
Seeing a webpage for the first time, users categorize regions at a glance. Certain design cues will suggest content to them (columns of text, navbars, form elements) and others suggest ads. Along with that initial glance, most users are conditioned today to look mostly in specific areas for the page content.
Here are the top design aspects that may cue users to think something is not content:
- Shape: Wide rectangles
- Position: Top of the page or on the right side
- Color: A different background color than the rest of the page
- Border: box blindness
- Different fonts from the rest of the page
- Unexpected animation or use of images
This post is really about designing content, not ads, but you can view some additional relevant pointers on banner blindness, animation avoidance, and pop-up closing from Jakob Nielsen.