I see where you are coming from here, but I think you are making some errors in your thinking. None of the players with which I play view XP as the 'finish line.' Most players don't. I certainly don't when I play. Getting XP is super fun and having a session where you get very little XP can be irritating, especially if you don't know why you didn't get much. But in the end, XP is just the gas your PC needs to accomplish whatever their objectives are. If I have a character that survives more than a few sessions, I always come up with some wacky scheme for them to pursue in the long run, for which XP is just the thing that helps them accomplish that goal.I'd like to hear how you came to that conclusion, because as I see it, you have it reversed. GP=XP I see as less useful for setting goals, because the players goals all become profit-driven.
Also, how can the party possibly know what treasure is involved in, say, figuring out who is haunting Farmer McGregor's field at night, or killing the giant feral hound that's eating village children? GP=XP party has zero motivation to look into that - Farmer McGregor is probably a broke peasant, and feral hounds aren't know to carry around gold and magic items. The party doesn't initially know they'll be rewarded with the Family Ancestral Sword, or that the feral hound has a potion merchant trapped in his den. They'll never know, because they aren't motivated to do the adventure.
Murder whodunnit? Nope, no money there, not interested. Tomb of Horrors? Nah, I heard there's not a lot of treasure to be had there. Why explore that dangerous old crypt when there's a dragon horde in the other direction with more money and only one real obstacle? And so on. Whereas if the party knows I award XP for completing significant events, then they're going to latch onto events, regardless of the financial gain to be had. If the party gets XP for killing monsters, they're going to hunt that feral hound to the ends of the earth and scour the Tomb of Horrors of all life.
You seem to be putting words in my mouth. What you think I call 'levels for nothing" is not at all what I'd advocate. levels for nothing implies... doing nothing. XP is given for accomplishments in a milestone system. The party has to solve the murder, or stop the bad guy, or rescue the princess, or whatever. That's not nothing; that's levels for adventuring... kinda the whole point of the game, really.
There's no expected progression because only I, the DM, knows when and how XP is going to be awarded, much like how only I know where all the gold is stashed if I were running GP=XP. The player's aren't privy to that - all they know is they'll see a goal and likely improve their characters if they accomplish goals with them.
In GP=XP, the finish line is GP. Guess you'll never meet a level 15 Druid who isn't a millionaire, or find a Level 12 Fighter who couldn't just hire an army to do the fighting for him.
In XP=XP, the finish line is undergoing the experience (hint - they are called "Experience points"). A level 15 Druid has seen some shit; he didn't just come across a treasure vault and suddenly gain new spells and abilities. A level 12 Fighter has wrestled with some serious foes; he didn't kill a single foe who happened to be fabulously wealthy.
Also, keep in mind that NPCs shouldn't follow the same rules as players. It's irrelevant how a level 15 druid NPC got to level 15, and he doesn't necessarily have to be a millionaire. A level 12 warrior NPC doesn't necesarily have to have an army. However, a level 15 PC druid probably spent lots and lots of money on his fancy garden or nature preserve and is quite pleased with his accompishments. A level 12 warrior PC should have an army unless he lost it doing something stupid.
Also, the game is more fun as a player if you know what you are getting XP for.
With that said, let me share my experience with different XP systems.
I've been DMing for about 5 years and I've used three systems for XP. The first system was not keeping track of XP and then leveling up the characters when I felt necessary. This is good if you are a total beginner and you are running Lost Mine of Phlandelver or something. This system also works very well if you have an episodic campaign where you switch DMs every few sessions, but other than that, it sucks and it has we don't need to go into that.
The current system I use is what you call XP=XP. At the end of each session, we do a quick recap of the session, during which I award XP. We currently play DCC so the experience points in the examples below much lower than traditional DnD. I award them 1-4 XP per based on three main things: risks, treasure and goals (sometimes knowledge too).
For risks, the bigger the risk, the more XP they get. If a PC actually dies, the survivors usually get the full 4 XP.
For treasure, If they find a hoard of treasure, they get the full 4 XP. If they find bits and pieces of valuable treasure here or there, then they get XP for that too, but not the full 4. This is on top of risk. So, if they sneak into a dangerous dragon's lair, find their way to the hoard, fill their pockets, and then craftily bounce, 8XP.
As far as the goals go, they either come from me (the DM) or the players themselves, and I award XP based on how well or how close they came to accomplishing a goal. For example, if Granny (an NPC) asked the party to save some asshole child who got kidnapped by gnomes, and they succeed, they get 4 XP. But if they only found some clue as to where the child was, then 1 or 2 XP. If they made up their own goals, then I give them XP based on how well I think they did on those goals. For example, if their goal is recruiting more cult members, then they might get 1 XP for a handful of villagers, but 4 XP for converting the local warlord and his warriors. Again, this is all on top of how big the risk was. So, if they negotiated through a 3rd-party to have the aforementioned asshole child released by the gnomes, then no XP for the risk, but 4xp total. If they launched a full on surprise attack of the gnome warrens without scouting it out and one of the PCs is killed, 4XP for that, plus an additional 4XP if they managed to save the kid.
Here and there, I might award for other things too, such as getting a valuable or interesting piece of knowledge, or making some particularly interesting choices in role-playing, but this doesn't happen every session and it's usually a small amount of experience.
This system works well enough for my group, and incentivizes the type of play I like DMing. I haven't explicitly told the players this is how I am awarding them XP, but I think they sort of get it, and they really seem to like it.
However, I have run a few games with GP=XP and I was blown away how it changed the game for the better. First and most importantly, it gives you an objective measure of how well the party did (rather than just how the judge felt about it.) This makes getting XP a lot more rewarding, and not just feel like a byproduct of showing up to play. Second, it incentivizes the players to take risks and look for secrets, since gold is usually well protected or stashed in weird places, thus encouraging your players too try to explore for all of the content you prepped. Third, it generally de-incentivizes combat and hunting down the 'BBEG'. I love tactical combat, but one or two battles per session is more than enough for me. The concept of the 'BBEG' that is omnipresent in mainstream DnD also really gets on my nerves. Finally, if you implement the Dave Arneson rule ( GP≠XP, but rather GP spent=XP), it incentivizes the players to take ownership of the world they are in, which is what drives long-running campaigns forward.
Yes, Gold=XP might make the game more about profit and it might make certain types of hooks harder to convince the PCs to take (like helping some crusty farmer, or investigating a series of dissaperances) but these problems are fairly easy to mitigate and you get a lot of benefits that no other XP system can offer you. Having the players know how to get experience points makes the game significantly better (in my opinion), and GP=XP is the cleanest form of that. (Monsters killed=XP being the worst, but that is for the adventure leagues goobers)
The only reason I don't do GP=XP, is because last time I asked my players about it, they said they would rather do what the DCC rule-book says which is '1-4XP per accomplishment in a session.'
Bunch of rule-following squares, really.
I should edit this before I post a giant block of text that nobody will ready but fuck it, I am late for work.