Go to the reddit homepage. Look at the very top link (that's highlighed with an outline). That's an ad, but it's hosted by reddit and contains no scripts or iframes, in fact doesn't even contain offsite images. Its div has the CSS class promotedlink. If you look in the default list used by Adblock Plus
https://easylist-downloads.adblockplus.org/easylist.txt you will see that it blocks this because it contains the line
reddit.com##.promotedlink
But Adblock Plus actually contains a second list of "Non-intrusive advertising" that actually allows ads on reddit. Many people are very angry about this list because many companies pay to get their ads put on it and allowed.