I like to keep it simple. Armor class is 10+ Dex Bonus + Fighting skill bonus, + shield bonus + magic bonuses.
INstead of a progressive Base Attack Bonus that goes up every level or so, all characters have a Fighting Stat that gives them their Base Attack Bonus.
Players have 1 hit die of hit points, plus Con bonus. Because they are heroes, I let them have half of their hit die, and they roll the other half. No one wants a fighter with 5 HP. So a d10 fighter has 5, plus 1d5, plus Con bonus.
In addition, all players have Player Points, which can be used to mitigate damage, buy rerolls, and cast spells.
Armor gives a reduction in damage, based on type of armor and quality of armor.
Any time a character crits or is critted, or is placed in a non defensible postion ( knocked off feet and pinned, grappled, snuck attack) this is called Real Damage, and Player Points can not be used in place of hit points. ( but can reroll the to hit rolls of the enemy etc).
This system makes it possible for low level characters to challenge more powerful monsters, and high level characters are still threatened by weaker monsters, which I feel is the way it should be.
Player Points start at 100, and they recharge 20 per day of rest.