X,Y,Z missing in F3

Minecraft 1.8.1 added a gamerule to show less information in the F3 information among the things made invisible are the XYZ coordinates, you should be able to disable the Reduced Debug Info by going to options, Chat settings and then toggling the "Reduced debug info" option.

If that doesn't work (can't test cause I'm at work) there is the gamerule reducedDebugInfo which you can set to false via /gamerule reducedDebugInfo false which should work for that world.

Create a Game and press Esc, then choose "Options" and pick the Chat settings. Then you can see the REDUCE BUGINFO: ON/OFF