Linear Build Quadratic EXP
So, you're sick and tired of all these goddamn D&D clones. You decide want to make your own game system.
And, you know what else, this whole system of class and multiclassing, that's bullshit! Why shouldn't players be allowed to make whatever they want, without restrictions from the game system? All this inorganic advancement shit is just pure powergamer-pandering!
It's a lot of work, but you go through with it. You nut up and build a game system that, at the very least, causes players to advance however they like by spending resources earned during play (like, you know, some kind of points gained by experience) to improve their characters, rather than locking them into set advancement paths. Good for you! That's actually a shitload harder to do from a game-design perspective than just building a bunch of advancement tables.
Then, you've got to price-out everything the players can "buy" with their points. And, usually, to keep people from just trying to do fucking everything at once, you probably made higher ranks of stuff proportionally more expensive. For instance, if it takes one point to learn a skill, you might need to spend two, then four, etc. to get better at it. Again, that's also fine.
But... now you've got to design character creation, and you might be in trouble.
See, now you're in a "damned if you do, damned if you don't" situation. Either you hand a potential-newb player a huge pile of "points" so that they can buy the stuff they want to make their character tick, and having to do that much math can turn them off... or you take a lazy shortcut and potentially wreck your game by using a different exchange rate for the points players use to build their characters. Usually by, say, letting them raise a stat or skill at a 1:1 rate for build points. It's quick, it's intuitive, it's a lot easier to do than working with a nasty, quadratic pile of XP.
And it opens a whole new can of worms.
Here's the problem with build points: like any lazy shortcut, they create long-term problems and drag in the old " Ivory Tower" newb traps. They heavily reward min-maxing, getting the stuff you want to excel at as high as you possibly can right off the bat, because it's significantly easier to, for example, raise a bunch of stats you left as low as you could get away with up to a point where they're no longer actively inconveniencing your character than it is to leave one of your "core" stats a single point below the maximum and try to raise it later.
In the worst cases, that experience deficit can really add up, essentially leaving one character as a guardsman and one as a space marine, despite both theoretically having the same amount of experience points, entirely due to the order in which they bought shit.
Worse, lots of "build points" systems let you invest them in material goods, like money or items, that can be used-up, expended, destroyed, or stolen during the course of the game. And then you're out even more experience points.
What's the moral of this story? (Aside from " houserule this shit"?) Well, bluntly, either come up with less-ridiculous long-term XP costs, or unify out your character-creation and -advancement systems, in the unlikely event you're making a game. Or you're just going to punish new players for the crime of inexperience, while actively rewarding the powergamers you got fed up with in the first place.
- White Wolf - All of their games have this problem to one extent or another, but Exalted 2E was easily the worst, due to its epic scale and byzantine rules. Basically, if you didn't have Dexterity 5, Willpower 10, and two Virtues at 5 at character creation, you were probably fucked without lube long-term if even a single member of the party did too, or the Storyteller would start having to struggle to present problems that left you relevant compared to your better-prepared friend. Third edition conspicuously refused to fix the problem, with one of the devs infamously declaring, and this is a direct quote, "We have a policy not to give people bad rules just because they think they want them." Holy shit.
- Shadowrun - Every edition but Fourth used a "priorities" system that was, arguably, an even worse version of this problem, and 4e, while it had a few handrails in place to stop this kinda thing, still held onto the BP system. Fifth edition, being a warts-and-all grognard edition, naturally brought back priorities instead, erasing even the mild progress this represented. Or not entirely, since they propose an alternative 'Karmagen' system that basically gives a pile of XP to upgrade your character with. Incoherent pricing of race/abilities/... sadly turn it into another problem altogether rather than fixing chargen.
- GURPS - ...Okay, admittedly, for all of the messiness and artificial complexity of GURPS, it does just give you a pile of XP at character creation rather than parceling out into BP and other stuff.
- Fantasy Flight Games' Star Wars - It does have the problem, but it deals with it in a somewhat creative way: once your core attributes are "set" at character creation, they are set, with only a handful of expensive and limited items and a couple powers at the very bottom of your class's power tree able to increase them. And the game deliberately tells you to raise them as high as you can. It's not a perfect system, but it helps.
- 7th Sea - ...Well, the drama die system has more problems than just this, but for the most part, it's remarkably stable at build. All five stats are important in combat, so it's hard to just pick one you don't give a shit about and leave it low, and the build costs, while about half of the cost of spending XP, are otherwise paid at about the same rate. The one exception, advanced knacks, are deliberately pricier at character creation and cheaper afterward, removing the issue altogether.