RealUI 8.1r17d
When you mouse over a world quest on the Map (press M), its tooltip isn't rendered properly. The background and the border appear under the map. Due to this, you cannot read the details.
The same does not happen when you mouse over a world quest using the Quest Log (press L).
You can see screenshots here:
http://imgur.com/a/9axIz
No errors are thrown, it just doesn't render correctly on the World Map. Disabling RealUI at the system level fixes the problem.