Since there's a new Stone of XP out, it got me thinking of some other possible unique ways to implement the "stone of XP" type of item...
Minor Stone of Experience
Grants you as much XP as it takes to advance to your next level.
It always grants the amount it takes to get from no-XP-progress to the next level, flat rate...ie, you don't "lose out" if you've already gained some progress towards the next level, it just bumps you up to level X with the same amount of "extra" XP towards level X+1.
They'd be available from the store (naturally), but also occur in rare drops and daily dice, about the same rarity as Tomes.
They'd be tradable, and usable at any level 1-20, so there'd be an economy around them, and it wouldn't be "wasted" when you find one on high-level toons.
Since the game is starting to accelerate the level gains as the cap goes up more and more, especially through Heroic levels, I think these would be a nice additional bonus. Especially for those levels everyone hates, 19 and 20. Plus, since they drop in-game, its not as completely P2W as store-only Stones.