Dimensional Doors appear to be buggy. It may be a permissions issue/ configuration problem, but attempting to go through one will likely result in your receiving an error message and disconnecting immediately after joining. The only remedy I have found is to reset player position in the player.dat file. At least two users have experienced this. Until we can find out what is wrong, avoid using DD.
Can confirm due to CZ walking into one after I told him 500 times to avoid DD. Though I haven't seen one in overworld - only in Mystcraft worlds, so when it comes to Mystcraft, - I would also warn against creating worlds without pages... that seems to cause issues too.
I am the reason why the overworld haz probs. I make the world shake when I walk Sent from my Space Cadet Mind