You know the deal: some website decides that it would be super slick to show a preview of linked pages when you hover your mouse over a link, or a preview of someone’s profile when you hover over their Gravatar.
The trouble is, it is obtrusive and distracting. Moving a mouse over a website shouldn’t cause dramatic new things to happen. When they do, they break concentration and cause frustration.
It is acceptable to have menus that enlarge when you hover a mouse over them. Mouseover text for images is also perfectly fine, since it takes a moment to appear and is thus effectively requested rather than imposed. By contrast, website elements that pop out of nowhere just because a cursor crossed something are bad form in the same way as universally despised pop-up ads.
Thankfully, many such unwanted features are killed by AdBlock and NoScript.