Come on, are stats really only going to effect how much health points and MP you have? These need to be greatly expanded. For example, to stop people from maxing every single skill available, there should be a stat limit on what they can and can't do. If you intelligence is level 100, you can't max more than two magic skills, say Crystal Way and Brown Way. Or, they could balance them, like level 20 Crystal, Brown, Red, etc. But being able to max every single stat makes your character seem like a God. In fact, these Ways shouldn't even be mixed. If you maxed Crystal way, you can't max Dark Way, or even train it. Instead you can only train relating skills, such as Azure and Blue ways to 3/4 of the max. And Red and Brown ways to only 1/2, since they aren't the exact opposite of crystal. Now if you max Blue way, you couldn't max Red way, and the balance would go on from there. But maxed magic levels should only be determined by intelligence. If you're dumb as a brick, say 40 INT, you could only get Crystal way to about level 15 at the most. To train it more, you'd have to train your intelligence first. Once again, this would stop people from maxing every single skill available and becoming Gods.
Here comes capacity. If your INT is maxed, and you max Crystal way and train a few other magic skills as high as they'll go, you'd have capacity problems. You can't learn everything in the world, or else you'd forget. If you have mastered Crystal way, and try to learn all other magic, you'd probably forget your Crystal way training, from trying to learn all those other things, and the skill itself would level down to make room for the skill you're trying to train. After all, we can't have Godly intelligence.
Now for fighting skills. Unless you're some kind of Godly warrior, you can't master all types of weapons. Maybe three at the most, and that's only if they're related to eachother, such as swords and axes, or maces and mauls. The way to fight with them would be similar, like smash the opponent or cut them. But to master using swords, holding the handle and slashing, then trying to learn pole arms, lengthing the pole to your arm and sweeping your enemies, that doesn't work. The weapons need to be related to eachother or else it just won't work.
Once again, the system works. If you master swords, you can master related weapons such as Axes. But you can't train the opposite, which would be maces or mauls. You could however, train polearms to 3/4 the max, and maces and mauls to 1/2 the max, but you can't max either one, since you've already maxed swords. You could however exceed the capacity and level down in swords to train them further (just a note: melee has nothing to do with this).
Now for stat limitations for fighting skills. To majorly stop powerleveling, you need to have endurance take a little more effect. This means if you've powerleveled for such a long time, you'd grow tired. This has already been implimented, with SP going into effect after long fighting or mining, but it takes far too long for the person to grow tired for that system to work. At this point, when your SP reaches 0, Will would come into effect. The will to carry on despite grevious wounds. If your enemy and you are both tired, the person's will would decide who would win. They could deliver a final blow by costing a little of their own HP, and kill their enemy. But after that they wouldn't be able to move, and would have to rest in that spot.
A few short descriptions of what I think skills should be:
Strength - Determines attacking power. Related with Will and Endurance.
Endurance - Determines how long you can do something, such as fight or mine. Related with Strength and Will. (CAPACITY)
Will - Determines what you can do despite being heavily exausted, such as take one more swing with the pickaxe or deliver a final blow. Related with Strength and Endurance.
Intelligence - Determines how strong your magic will hit, along with how much magic you can learn. (CAPACITY)
Agility - Determines your quickness, gives you the ability to run faster, jump higher, and deliver a quicker blow.
Charisma - (not decided)