We're about fourteen sessions into my Mage: the Awakening chronicle, and I've been pondering the game's system of distributing experience points. Mages receive the normal share of experience that all World of Darkness characters receive ("general experience"), as well as earning a secondary pool of Arcane Experience. The stated reasons for the existence of Arcane XP are interesting: firstly, mages have to be out of their sanctums exploring the world (including magical worlds beyond our own) to earn Arcane XP. That is to say, they can't just hole up in their sanctums, use sympathetic magic to screw with their enemies, and remain unthreatened. I'm fortunate that my players don't seem too inclined to do this. Well, maybe one of them, but the rest of the party can be relied upon to find some kind of trouble.
The second reason, to keep the mages on par with vampire and werewolf characters, has not the least bit of bearing on my chronicle, but I'm glad they included it. I have played in a nWoD crossover game, though I don't recall at this point whether or not the mage in that game got Arcane XP.
Since my players aren't dependent on Arcane XP to motivate them or to keep them balanced with other supernatural creatures, Arcane XP are just experience points that they can only spend on Gnosis. I doubt seriously that this is adding anything to the game other than keeping them from having to choose between the next level of an Arcanum and the next level of Gnosis. With only a few exceptions, I think they've come to feel like their only meaningful options for spending XP are their two Ruling Arcana. (If they're close to accumulating enough Arcane XP for a level of Gnosis, they might consider dipping into General XP to finish that out, but it's not a priority.)
This mindset is not great for me, as the Storyteller. Every new dot in Arcana is a major leap in their capabilities. New dots in Gnosis are also significant steps up in power, though at this point not as significant of steps. In theory, the game offers a vast list of other options for purchase: attributes, skills, skill specializations, merits, rotes, and so on. In practice, however, nothing on that list is going to offer anything nearly as interesting or useful as Arcana. On one hand I acknowledge, "Rightly so," because... c'mon. They're mages. If improving their magic isn't right at the top of their priorities, something has gone badly awry. On the other hand, I'd like for them to feel like they have more than two options, and that it's worth investing a few experience points in that huge list of skills rather than looking at me dolefully when I ask them for a roll using one of their (many) untrained skills. Increasing the breadth of skills used doesn't really help: it increases dolor, without changing their minds on how they spend experience. (Since my players will read this, I recognize that I'm making generalizations that aren't quite so absolute in actual play.)
I've been pondering a change to this system for a long time, out of a simple discomfort with any system in which the XP decisions seem easy (other than "which of my Ruling Arcana will I increase - I've watched them agonize over this decision for multiple sessions), and to a lesser degree a desire to make them more comfortable with other kinds of purchases.
The change we've discussed would be to make Arcane XP the only way to purchase all Arcana and all Gnosis. I would probably shift the balance of experience awarded; where I currently hand out 2-5 General XP and 2-5 Arcane XP (depending, more or less, on how much stuff the PCs did in that session), I'd start handing out... I dunno, 1-3 General and 4-7 Arcane XP, to avoid completely stifling their advancement in Arcana and Gnosis. As they're nearing their fifth dot in Arcana and their fourth dot of Gnosis, prices are getting pretty brutal.
Naturally, the point of an cost-scaling system is that eventually you get to a point where cost-benefit analysis makes you start looking elsewhere for more immediate gratification. Also, the fifth dot in an Arcanum is the cap until they reach Gnosis 6, which gives me a decent amount of time in which they are merely masters of their Paths. I'm not too sure what the game will be like once they reach that point; their rise to power so far has been pretty meteoric, in my conception of how long it should take to be powerful enough to challenge significant, named characters in the Boston Consilium. I feel like the designers assumed that players would build up more slowly than this, given that the game's actual rules kind of... stop... at Arcana 5 and Gnosis 5. I mean, I know what happens with player stats past that point, but the game makes not the least shred of effort to define in rules terms what kinds of magic one can work at those levels.
It would be like 3rd edition D&D's Epic Level Handbook saying "Fighters get this, rogues get this, mages are just goddamn ridiculous and can do whatever they want if it seems cool at the time, and the same goes for clerics." For all of its problems, the Epic Level Handbook tried to do better than that by at least providing guidelines for building newthings. (One of the more interesting pure-crunch design exercises I've ever read was a couple of very smart people breaking down the Epic Level Handbook spell seed rules and looking for broken stuff.)
Anyway, back on topic: this is one of the essential problems of any classless build-based system. CI/RBP LARPs, which are certainly classless build-based systems, have an easier time rewarding skill dabbling, because you can't always be sure that there will be someone around who has any given skill. Some things you really do need just for yourself, such as defenses. Were this a purely-mortal World of Darkness game, I'd be having a similar problem with fighters not really having to buy anything but more dots in combat-applicable stats, and I expect strongly that my two really combat-focused PCs will just finish buying up their combat stats if I make a change like this. Having them do that, of course, is only a problem for me insofar as I want combat scenes to challenge everyone, rather than being too hard for the low-combat characters or too easy for the fighter types.
This is a good opportunity for me to mention my biggest problem with WoD games in general: the games give extremely little guidance as to how to turn the presented rules into an exciting challenge. The problem with totally abjuring tactics in favor of roleplay is that it leaves both the Storyteller and the players adrift in figuring out how to make the fight exciting without introducing huge amounts of ST fiat.
In summary: classless build-based systems can be haaaard. I have an obscure feeling that there are further issues that I'm overlooking - possibly things that would point out to me that this isn't really a problem, just an inescapable phase in any Mage: the Awakening chronicle. But, see, I've never played a PC in Mage, and Harald is the only other active Mage Storyteller I know - and I'm pretty sure he's mentioned ignoring Arcane XP completely in the past.