I also sort of think they aren't - there aren't any really easy to do silver challenges from a newbie / casual perspective. As I've moaned about incessantly - I still can't figure out a way to unlock the assasins dagger.
Second thing about quest items is that there are many different categories and no obvious simetry. For example, there are no "mis-directional" dungeon "4 class comletion" unlocks for the north and the south, and the east and west both unlock magic user gear (Battlemage Ring and Mage Plate if I'm not much mistaken).
Also, I'm using the quest item prep for both platemail and tri-sword users (when I'm not prepping them), soul orb/viper ward scumming, battlemage ring / MW / Mage Plate guys (biggest focus on magic gear), and it also includes Martyr Wraps and Gloves of Midas AND a whole bunch of really niche stuff. It's a bit of a mess tbh.
I see a whole bunch of approaches to putting some order into it. One would be to have the first few items anyone unlocks through questing (or rather, the items unlocked in Venture Cave and Den of Danger) expand the "basic" pool rather than be Quest items. Then figure out how many quests would that need and what the "complete" pool would be (also define the quests for Venture cave - I'd love to have 4 questilnes for eah class building simmilar to Unlikely Heroes).
This would make the standard pool a bit more "embargo" proof to discourage, erm, enterpenurial minded individuals like myself from overly easy shennanigans.
Then I'd deffinitely decide upon what items do Slime Pits, Cursed Oasis, Magma Mines and the Berserker Camp give out for "Complete with X classes" quests, figure out what category they go in and have them be decent items for various approaches (Battlemage Ring and Platemail come to mind, not sure about the other but Crystal Ball and Fire Heart could be nice once they are "fixed in place"). They could also increase the basic pool just with less chance of appearing, sort of "cap it off". Or not.
And then have the Challenge Rewards include the Puzzle Pack rewards (Soul Orb and Viper Ward) for a good mix of general-purpose / niche stuff for what is now the "Quest Items Prep" - people would still be taking it for Martyr Wraps, Gloves of Midas and so on and so forth. I guess.
There really ought to be a thread about this.