I've noticed a trend lately where sites (blogs and news sites in particular) will put up an overlay the first time you visit their site, asking you to sign up for an account or take a survey. They almost always have a little "X" button, or a "Close" button, or something to close them.
These sorts of overlays are in line with pop-up ads; they demand the user's attention, and block usage of the site until they are dealt with.
Worse, for malicious sites, closing the overlay can be used to clickjack the user. See https://en.wikipedia.org/wiki/Clickjacking.
I've gotten in the habit of deleting the overlay's DOM elements using the Chrome console, to prevent any sort of clickjack attack on unfamiliar sites. I've been looking into better solutions today, and have found a few plugins that might make the process easier, but I'd like to put these sorts of pop-ups forth for consideration as a form of intrusive advertising.
I think it's within Adblock Plus' purpose to block these sorts of pop-up overlays, even though they usually come from the site and ask the user to do something for the site they're visiting.