Basically, there is a dearth of XP available after the Vale, mostly due to there being very few quests in the 16-20 range, but also because many of those quests offer a very small percentage of the XP required to advance at that level.
What I mean is that, at level 14, let's say, running a Gianthold quest in 30 minutes for 6,000 XP is worth much more, comparatively, than running an Inspired Quarter quest at level 18 in the same amount of time and for roughly the same amount of XP, because the amount of XP required to get from 18 to 19 is much, much greater than to get from 14 to 15.
One solution, obviously, is to add more quests in this level range, but that takes time.
The solution I'm suggesting is to increase the XP in these quests to represent a bigger chunk of a percentage of that required to advance a level. I'm looking at the 500,000 XP required to get from 18 to 19 on a double TR, and looking at the amount of XP most quests in this range are offering and am getting incredibly discouraged.
Yes, a couple of the Reaver's Refuge quests are worth a lot of XP, and the Vale quests are worth a ton on elite (more than almost every other individual quest that comes after them), but the grind is ridiculous. Getting from 19 to 20 is going to be really awful, though there will at least be more options due to the availability of both the Dreaming Dark and Amrath quest lines.
At the very least, this could be used as a stopgap measure until more content is released: increase the XP on these quests by 25-100% now, then when we get more content dial the XP back (though they should probably be worth more in the end than they are currently anyway).