Last week The Old Republic revealed some of the game mechanic changes coming in Fallen Empire. The most controversial is that all planets will be level synced. That is, when you out-level a planet, your character and gear will be scaled back to the max level for that planet.
On the one hand, I can see why some people are unhappy. These games are about character progression, and level syncing undoes your progression. It's fun to come back to an old zone that gave you trouble and destroy it. What's the point of gaining levels if the game is going to arbitrarily roll them back?
On the other hand, because it's normal for a max level character to overpower old zones, the game never sends you back to those zones. Or if it does, it sets up a little max level area where the max level players go. It's harder to preserve challenge without level syncs.
Level sync makes it easier to add new content to old zones. For example, all holiday events in FFXIV are level 15 or 30. High level players will get synced down, and the low level requirement makes it easy for new players to join the event.
However, FFXIV only scales group content. Solo-content is pretty much left alone.
Another point is that level syncs also balance world PvP a bit better. Sure, a synced player is more powerful than a leveling character, but the disparity isn't as great any more. If world PvP starts up, the leveling players on the planet can join in.
In the end there is an unavoidable tension between maintaining character progression and preserving challenge. I think that FFXIV strikes a good balance for group content. I thought that Guild Wars 2 did a bad job maintaining character progression, as it was very aggressive about syncing levels, so you'd get scaled back even in the same zone. Games without leveling syncing basically give up on attempting to preserve a challenge in old zones.
We'll see how well TOR manages to balance both goals.