Or rather, the lack of an economy. In 4E, there is very little for players to spend their treasure on. Consumables like rations and ammunition and ritual components are about it unless you allow the purchase of magic items, which I don't except in VERY rare cases (Ghul D'Khat once managed to purchase a magic sword, but this was a plot device that ended up shaping the character in more ways than one) with the exception of scrolls and potions. In AD&D, there were reasons characters needed to keep risking their lives for golden hordes.
1. The biggest of these was that you had to pay out cash to gain a level! I think the amount listed in the AD&D DMG is exorbitant and I never gave out that much treasure for the most part, but I think that's a great rule to introduce a little hunger back to the PCs and give them a reason to seek out treasure. I'm not exactly sure where I'd set the bar, but the treasure parcel system could serve as a guide in some manner, requiring a percentage if the total expected monetary treasure.
2. Henchmen and Hirelings were an integral part of the original design of the game that we never really took much advantage of. Men-at-Arms were ridiculously cheap though normally came with the caveat that the PCs had to purchase their weapons and armor. Henchmen were relatively expensive and often seemed to live better than the PCs. Recently WotC put out an online article for incorporating these kinds of NPCs in 4E and I liked the basic concepts they presented. If I do ever try to run 4E again, I am going to make sure to include the options presented there.
3. Mundane upkeep was not an official AD&D rule, but it's one I always used, even in my 4E games. Basically, characters were required to spend 100gp per level per month. These costs covered the free wheeling lifestyle of adventurers - private rooms at the inn, three meals a day, rounds of drinks in the taverns, stabling for mounts, maintenance of armor and weapons, and so on. A character might be forced (or purposefully choose) to 'slum it', needing only 25gp per level per month, but would be sleeping in dormitory quarters, missing meals, neglecting gear, etc. Such a life style might leave the character open to theft of property, a chance of catching a disease, a broken weapon on an attack roll of a "1", or other unfortunate event. A character choosing to spend 250gp per level per month might be renting a house with servants, throwing lavish parties, and have a tailor on retainer (and may also draw the interest of the thieves' guild).
Next time I will continue with these thoughts, but focusing more on the acquisition treasure rather than the spending of it.