I think hp as currency is a bit reductive. Let's talk about daily resources instead. That covers hit points, spell slots, barbarian rage, daily powers, et cetera.
In this model, daily resources are the "cost of doing business"--what you spend to keep yourself alive and victorious during an adventure. XP, meanwhile, is "investment," permanently expanding your capacities. Gold in 3E and 4E is a slush fund which can be spent on either. You can spend it on consumables and spell components which supplement your daily resources. Depending on the campaign, you may also be able to spend it on permanent magic items, which supplement your XP.
Gold is notable for being less tightly controlled than the other two, and also for the fact that it can be stored up. 3E demonstrated the problems that can result from this. On the one hand, wands of cure light wounds, knock, and the like could radically expand one's daily resources. On the other hand, permanent investment in the "big six" magic items had a staggering impact on character power. 4E tried to tackle this by instituting tighter controls on cash flow and having prices scale up radically. The results were... well, as a game mechanic supporting D&D the squad-level wargame, it worked okay. But when you got out of the dungeon and let PCs interact with non-adventurers, it started to break down.
Another thing about gold is that, in 1E, it wasn't a slush fund--it was just XP by another name. You found gold, you got XP. After that, aside from buying a few spell components, there wasn't a whole lot to do with it. From what I gather, the general idea was that you would spend it partying like a rock star. (I may go to the May art thread and ask Jon Schindehette if we can get an illo of adventurers with a sack full of gold pieces making it rain.)
I think daily resources and XP are going to look about the same in D&DN as in editions past, but the role of gold is an interesting question. I'd like to move away from the "slush fund" model, and I'd love it if the rules encouraged or at least did not punish partying like a rock star. However, I also don't have any great hankering to go back to gold-as-XP.
Ideally, gold would be the currency of "interacting with the campaign setting." It wouldn't do much for you in a typical adventure, except for the occasional bribe. But if you want to hire servants, buy a castle, raise an army, or just throw crazy parties, gold is what you need.