One of many features Hack/Mine introduces to Minecraft is a stat system. It currently includes Strength, Agility (now defunct), Dexterity, Intelligence, Wisdom, and Fortitude. While the mechanics are mostly in place, the balance still has a long way to go (That's what alphas are for!)
 Leveling and Experience
Each player begins at level 1. By killing monsters, players gain experience, which in turn increases their level. The experience required to level up increases with each level (and was more than quadrupled with patch 0.5.) The planned level cap is level 30, though currently there is no limit in place.
Currently, all experience given by a defeated enemy is based on whoever dealt the killing blow; this will soon be replaced with a more fair, damage-tracking system, but for the time being makes for no experience when the mob dies from fall, lava, or poison damage.
Players do not lose experience, levels, or items on death (though they will take a 10% durability penalty to all items in singleplayer).
With each level up, the player gains 5 points to use to increase their stats. If points are not spent between level ups they stack up for later distribution.
 Stat Mechanics
Strength increases melee damage by .5 per point. It may soon affect things like mining and digging efficiency, among other manual processes.
Agility is currently not implemented in the game. It may be added in a future update, possibly gaining some of the numerous bonuses from Dexterity.
Dexterity increases Defense by 1 per point (which translates into by 0.25% damage reduction per point), ranged damage by .5 per point, Attack Speed, and Crit Chance. Crit Chance is currently the only stat to feature diminishing returns, but Defense is soon to follow (the current system is silly in that it allows 100% reduction at 400 Defense). Dexterity currently affects a lot of things, so splitting it into both Dexterity and Agility is an ongoing possibility.
Intelligence increases magic damage by .5 per point and general spell ability (Currently, just the number of projectiles emitted by Charged Bolt.) At the moment, the only things that constitute "magic damage" are the Wand's projectiles and the bonus elemental damage on weapons. The Mage's spells also increase damage with intelligence, but at a differing rate.
Note: It may be feasible to build an "Elemental Warrior/Archer," stacking Intelligence and elemental damage, rather than Strength/Dexterity and physical damage. Only worth doing right now if you can stack more elemental damage than physical (which would be difficult), but once mobs have elemental weaknesses in place, this may become an extremely effective, though difficult, class build. For similar reasons, Mages may be able to use bows and melee weapons with elemental properties very effectively (and gain even stronger boosts from elemental buffed wands. This would be very broken, except direct damage buffs are possible as well. It just means the Mage has an easier time finding stronger gear.)
Wisdom increases total mana by 10 per point. Note that the rate of mana regeneration is a fixed percentage of total mana.
Fortitude increases total health by 12 per point for the Warrior, 7 per point for the Mage, and 10 per point for the Ranger. Note that the rate of health regeneration is a fixed percentage of total health, 3% of the Max HP rounded down at the decimal.
 Stats and Spells
Though Intelligence is currently the only stat to directly improve any spell's ability (see Charged Bolt), the other stats are being considered. Strength, Dexterity, and Intelligence all increase their respective damages indirectly, though. For example, the Warrior's Charge damage is based on his melee damage, which is partly based on Strength.
 Point Distribution
Currently, Strength is a completely useless skill for the more advanced player, who specializes in bows and wands rather than swords, despite choosing Warrior as their class build. Dexterity is one of the better stats for rather obvious reasons, but so is Intelligence, because of the added damage when using weapons (or armor, in the case of the unique item "rugged cross") that stacks. Regarding the damage bonuses from intel, a player sporting a wand with 3 or 4 elemental damage types (These are very, very rare and require large villages with numerous vendors to acquire in a reasonable amount of time) can expect to see their damage per point increase by 2 or 3, versus the puny 0.5 rewarded by Dexterity and Strength. Dexterity is also best used for its defense, which grants invincibility by 400. It seems like a long stretch but is actually fairly easy for experienced players to achieve. Note that things like poison and fall damage do bypass the damage reductions granted by high def. Fortitude always helps but is rather low priority, as berserkers tend to be more effective in Hack/Mine. All your health should be gained through high level rings, amulets and armor. Wisdom is only necessary at very high levels. The build idea is to build pure dexterity until 400 defense is achieved. Use high level bows to deal easy, massive damage. After this point you want to train up intelligence, which is where wisdom comes into play. Once your spells cast 8-12 mana per shot, you want to get your wisdom up a bit -- say 1000 mana without armor and 1500 with. This cycle should repeat, with each shot costing 1% of your overall mana-high dex allows you to shoot very fast!