Scarcity creates desire
My son is watching Kinder Surprise unboxing videos on YouTube. Putting aside why he is watching these (answer: he’s four and doesn’t discriminate much yet), the mere existence of such videos caused a moment of perplexity in the house. Being Canadian, we don’t really get intuitively why Americans are so obsessed with Kind Surprise, but the simple explanation occurred to me: they’re valued because they can’t be had easily in the States. 
Which leads to the gaming insight into why old-school play can be so compelling, and a bonus insight about the state of the modern rule sets.
Players most want what they can’t have right now. They want that next level. That first magic item. That first big haul of gold. That brass ring, figuratively and literally, can be an incredibly compelling motivation to dare the forbidden places of a game setting and risk (the character’s) life and limb therein.
Scarcity creating desire is well-known. It’s part of why diamonds and gold have historically been worth more than other bits of rock. (The rest of the reason is that they’re particularly shiny. Humans are creatures of simple pleasures.) Manufactured scarcity is the business model behind diverse companies from Disney (their Vault policy) to Wizards of the Coast (rares in Magic: the Gathering boosters) to De Beers (diamonds really aren’t very rare anymore, so they throttle supply). Scarcity in games is a huge motivator for human behaviour, and this remains true even in the shared consensual hallucinations that are the central activity of the roleplaying hobby.
If you want to motivate your players, make things scarce.
I’ve seen this in my own play lately. Players that were hard-core WotC D&D players have suddenly been lusting after treasure and XP in ways that I’ve never seen. The need for creative approaches is bubbling to the surface. A scarcity of hit points is motivating working hard to keep those HP, and the desire to avoid danger. This tension between seeking treasure and yet trying to avoid the inherent danger of the places where treasure is found is, as far as I can tell, the quintessence of a certain kind of old-school play that is immensely enjoyable.
Conversely, if you want to demotivate a behaviour, remove the related scarcity. This is a large part of why modern D&Ds have come to where they are: in the quest to satisfy consumers, more and more player desires have been answered by removing scarcities from the game. No longer is it hard to get XP, treasure, or even magic items (they’re parcelled up in convenient bits and hidden inside encounters that are designed to be easy-but-not-too-easy to overcome, much like the toy in a Kinder egg). No longer is high level something to aspire to: it’s a given that your character will survive to that high level, assuming everyone maintains interest in the campaign. No longer is fun something to be sought and made yourself, but it’s something every designer is scrambling to somehow guarantee in the rules themselves (and have yet to figure out how to deliver with perfection).
I suddenly see why dyed-in-the-wool old-school gamers often accuse “new school” gamers of being entitled and spoon-fed. Straddling the old and new, I don’t see it that way; but I do see that new-school games must be understood as either “failed” designs or entirely different games with different design goals. (Hint: the latter is the correct answer!)
But what does satiety accomplish?
Like Maslow’s hierarchy of needs, if you satisfy the basic game scarcities, players are freed up to start taking those for granted and moving on to satisfying other needs.
What satiety does is something a designer needs to consider. It’s a game after all, there needs to be something to strive for if it’s going to be more compelling than Candyland. If you’re going to make character power a given, what does that free players up to strive for? Is that what you want your game to be about? If you want your game to be about charming rogues delving dungeons, handing success in that endeavor to the players will make your game goal different than you intended.
One significant danger is that satisfying one scarcity merely moves the location of scarcity up an increasingly-narrow pyramid of player desires: what’s at the top? Would a game that satisfies all player demands actually be a game still? 
I need one more space to get an RPG blogger bingo, so let me leave off with the question: What does that mean for DnD Next? Will 5e be able to make different scarcities simultaneously scarce so that 4e players can enjoy their tactical success scarcity while TSR-era players can still enjoy their survival and treasure scarcity? I can’t imagine how that would work, but I’m curious what Mearls and company will try.
For those of you who have no idea what I’m talking about: Kinder Surprise are hollow chocolate eggs with a toy inside. They’re available worldwide, but not in the US because the FDA forbids foods that entirely enclose non-edible objects. They’re definitely not fascinating to adult Canadians, and yet they remain fascinating for adult Americans.
This is why I think so many (but by no means all) old-school gamers and modern-D&D gamers are joined in reviling “indie” and post-Forge game designs: those games remove scarcities that they value as core to their conception of gaming. They moves the scarcity to somewhere else (there’s definitely still scarcities in such games), but to a location that such critic simply can’t see or doesn’t value as a motivation of play.