I just finished World Completion and got my 2 Gifts of Exploration. However, the "Been There, Done That" achievement is at 99/100 and my World Completion percentage is at 99% with 300/303 Renown Hearts completed. What am I missing, here?


I just finished World Completion and got my 2 Gifts of Exploration. However, the "Been There, Done That" achievement is at 99/100 and my World Completion percentage is at 99% with 300/303 Renown Hearts completed. What am I missing, here?


With the September 2014 Feature Pack, 3 new Renown Hearts were added (though, not mentioned in the patch notes). If you have previously completed the zones that the Renown Hearts were added to, then you will still be able to receive your Gifts of Exploration upon map completion. However, the full world map will still be technically incomplete.
You can find the new Renown Hearts in Caledon Forest and Plains of Ashford. The hearts are close to the entrances of their nearest city, meaning that the new Renown Hearts in Caledon Forest are near the entrance to The Grove and the new Renown Hearts in Plains of Ashford are near the entrance to Black Citadel. The hearts will not be visible on the map until you speak to the Scout near the entrance of the city you are coming from, even though you've already explored this area.


The thing is that Renown hearts are invisible on the map. Once you discover them, then you can complete the tasks involved to complete the Renown Heart.
Seeing as you're missing 3, it looks like you need to do some more exploring.
From the wiki:
Hearts are invisible on the map until you discover them. You can discover them by talking to a nearby scout, or walking into its range.