Each Class allows you to spend XP to increase certain stats. Different classes have access to different stats — Warriors get HP and weapon skills, Scholars get MP and magic skills, etc. Upping a stat increases its XP cost, so you can’t just overlevel your HP into the stratosphere.
My original plan was to include every stat as raisable in some way, but it turns out this is rather problematic. There are a number of reasons why it didn’t work out:
Small Numbers Result in Big Power Increases
I’ve intentionally kept numbers low to make them more instinctively understandable. 20 is a lot of damage in this game, whereas in other games 2000 damage might be a weak hit. This approach has a drawback — if +1 Physical Attack is a big deal, you can’t just give out that +1 willy nilly. Most humanoids start out with 3 Physical Attack. If the player rushes to spend their XP on another +3 Physical Attack, suddenly they do twice as much damage as the baseline! They can easily trivialize early-game encounters that are balanced for much lower amounts of damage.
Of course, the player does make some sacrifice when they spend all their XP on their stats. Which leads me to the next problem…
Raw Power Increases are Less Fun than Interesting Abilities
XP is spent on three things in this game: Abilities, Passives, and Stats. Of the three, the one that changes how you play the least is Stats. Abilities & Passives are interesting to learn and master. They give you new ways to approach problems, or change the risk/reward of existing strategies (do I go into dangerous territory to hit this guy from behind so I can benefit from my Backstabber passive?)
Stats generally just make the same strategies better. They may be briefly satisfying, but there’s nothing new to learn or experience now that your fighter does twice as much damage.
It’s not that being able to increase a stat for XP is bad, it’s just not as fun as the other options. You might think players won’t choose it because it’s less fun, but, unfortunately…
Power Increases are Almost Always the ‘Best’ Option
There’s a reason most ‘optimal’ builds in games boils down to: take the +10% damage talent. Take the +10% armor talent. There’s just not much that can beat a flat upgrade to effectiveness.
If the power increase is big enough to feel worth taking, it’s likely better than situational options. This is why you don’t find the Weapon Focus feat in newer editions of D&D. (Weapon Focus is what they call a ‘feat tax’: something so good you’ll end up getting it no matter what.)
Similarly, this is why you find a lot of RPGs with frankly underwhelming upgrades, especially in Action RPGs. Like leveling up the Soldier’s Assault Training in Mass Effect gets you an entire… +1% damage? Great… really exciting.
It may be balanced, but it’s practically invisible to the player.
Anyway, for me what it comes down to is that increasing core stats imbalances the game and is not as fun as spending the XP elsewhere. So, now you can only increase 4 kinds of stats: HP, MP, and Skills, and Movement.
HP and MP are in because they generally only increase your effectiveness by a linear amount; more MP to cast more spells is useful, but if your spells aren’t good enough in the first place then more casts won’t help. HP & MP also come in larger increments than other stats, so a +1 here or there isn’t a big deal.
Skills are in because they are important to customizing your character. If you want to make a Greatsword-wielding rogue, you can. They also offer a mildly interesting trade-off between specialization and generalization. You aren’t necessarily playing a worse character if you don’t increase any skills, but Attack Power is always relevant.
Movement is in because it actually has a big influence on strategy. I’m planning on doing a writeup on different movement ranges and the implications it has on tactical combat. Suffice it to say that Move distance has strategic implications, meaning it’s both fun and effective to increase. It also becomes prohibitively expensive before it would present balance issues.
EDIT: In the end I decided to use a separate ‘currency’ for stat upgrades besides XP. The lessons learned — and the learnable stats — from above still apply, though.