Just to make things absolutely clear:
Loot from chests is based on your level
and the 'level' of the chest, which has been individually assigned. This means that a chest may have been set to contain Epic-class loot and thus will always contain Epic loot, but the Epic item in question will still be of lesser quality if you open it at level five rather than, say, level twenty. In other words, it has been previously determined whether a chest will yield loot that is poor, okay, great, stellar, etc., but just how poor or just how stellar it will be, will also depend on your level.
Quest rewards offer fixed amounts of XP and/or gold, plus a variety of random items to choose from, like you said. But again, 'random' isn't absolutely random, because what types of items may appear has been manually assigned. And once more, if the reward is for instance a Heroic-class helmet, its stats will be better if you finish the quest on a higher level.
In the Flames of Vengeance part of the game, malachite gems have been added to some of the possible quest reward choices, so if you're lucky you may take home five or six at once for solving a single quest!
