EASY solution for people who want a slower and more challenging questing experience.
Self-imposed nerfs!
Choose in increments of 5% or 10%. Lower XP gained. More damage taken from enemies. Less damage dealt to enemies.
You could have a vendor somewhere, similar to the ones that lock your XP gain, but give you nerfs instead. It's the perfect solution for players like me who want to slow down the leveling experience!!
WHY NOT!!??
Now someone go post on the official forums because I'm not a subscriber and can't make posts.
EDIT: I want to know how it affects YOU, how it affects your experience. How does that make your gameplay experience worse if there is an optional vendor that will give a player a nerf if they want it? And since it doesn't, why NOT have that as an option for those of us that want it? It's probably VERY little work for the developers to implement something like this, but it would go a LONG way to making some of our experience so much better.
And to those saying to equip lower gear, that's much more complicated than a simple toggle. And that still doesn't affect my XP gained from other areas.