I've recently had some of my friends try FFXIV, and I had a very difficult time trying to enjoy the experience with them due to the level difference. I know I could start a new character but none of my friends started at the same time so I'd have five new characters that I don't want to deal with. I love how Guild Wars 2 level syncs everyone depending on the location so a max level player can play in a starting area with new players and not be overpowered. I think FFXIV should have a similar system for our overworld. In our case I think making it an optional setting would probably be better as some players are probably used to grinding mob dropped mats quickly in low leveled areas.
The issues gets kinda resolved once my friends reached dungeons, though I'd love to be able to at least smack an overworld mob with them without one hitting it. I want to be able to do the overworld content together (more then just fates) without making a new character every time a friend tries the game.