Jumping into Final Fantasy XI with level sync as a viable option is like jumping into an entirely brand new game. Level sync is like City of Heroes sidekicking, but on steroids. All you have to do is select a member of your party and tell level sync to set to their level. That's it! Any member of the party who is above the level sync level will be automatically dropped down to that level, and anyone underneath the level will stay the same. So if you sync to the lowest member of the party, all party members are now the same level.
What makes this an even better addition is that items and equipment drop down with you. No need to go out and get a brand new set of gear to match your new level. At the same time, the team didn't want level 75 players to automatically drop down to level 20 wearing their 75 equipment and have the best stuff. The armor and weapons will decrease to the median statistics for the level, meaning if your buddy is a level 20 and is wearing level 20 stuff, he will have the better gear. But, your gear will still be totally good for kicking clippers and pugils back into the sea.
Plus, if all of this didn't sound good enough, you still gain 100% experience in level sync mode. All of the experience you accrue goes towards your higher level, so you can party with a group of level 30s and still gain experience to get you from level 39 to level 40.
The impact this has had on the game is immediately apparent. The Bahamut server had more people on than ever, including 110 people in the Qufim Island area alone. Grouping has gone from being a long chore to having extremely fast pickup times and enjoyable results. The only problem right now is that level sync can only be used if your party is level 10 and above. While this means that soloing is still the best way to get to level 10, it would be nice to have that option available even at the lowest levels.
A great new feature like this -- one that allows you to play a very group based game with your friends, no matter their level -- makes the game into an epic adventure that mimics the Final Fantasy series rather than just being another MMO knockoff. If you haven't played FFXI in a while, or were considering picking it up -- do it. Level sync is just one way this game has been revitalized for modern day play.