Despite the technological sophistication of current advertising techniques, the way online ads are delivered is in many cases intrusive, distracts users from their task, and annoys them. To delve into the reasons that make an online ad annoying, we investigate eight specific ad functional features and their effect on perceived annoyance: ability and ease of closing the ad, coverage of page contents, coverage of browser window, ad expansion, automatic ad activation, video/animation, sound, and ad targeting based on the recent browsing history. The study addressed 132 computer science students, both undergraduate and MSc, who were asked to select and document three to six online ads they consider annoying, resulting in a total of 462 recorded ads. The majority of collected annoying ads were automatically activated ads, which in most cases could be closed or stopped easily. Furthermore, ads that covered most of the browser window scored the highest percentage of perceived annoyance. Ads with video or animation and automatically triggered sound, even if displayed in a fixed size area, seem to significantly affect users’ perceived intrusiveness and attention distraction. Also, most ads reported as very annoying were not personalized and were automatically activated, while student level makes no statistically significant difference.