Since you asked, here you go. I broke it out into a separate thread instead of burying it several pages into the other one.
In the past, SSG has said that they want to discourage players running the same quests over and over again. That was the stated reason for permanent ransack of optional XP and a couple of other changes. There are several reasons that players run certain quests and avoid others, but one of the big ones is the amount of XP they give. There are some quests that are so good I run them every life. Others are even better, and I farm them every life.
The problem is that when a quest is released, SSG makes their best guess as to how much XP it should be worth, and it stays that way forever, barring a rare manual xp adjustment like we saw a few years ago. There's no way to get the numbers right when you do that, especially since the "correct" number can change over time. The solution is to have variable XP. Keep track of how often quests are run. Each week (or whatever other period you choose), quests that are run more frequently than others get an automatic reduction in XP. Quests that are run less frequently get an automatic increase in XP. The farther away a quest is from the mean, the larger the adjustment. Eventually, the numbers should steady out and you'll find that people run a larger variety of quests.
Yes, there are some problems that would have to be overcome. For example, you might want to exempt the last two releases as well as the last expansion, since people have to run those a lot if they want the best gear, and it would suck to get no XP while you're doing that. You should probably also lower the overall XP needed to level since there won't be any more quests that give really outstanding XP that you can use to level faster. I think it's worth considering, though.