I'm going to eschew the standard review structure of gameplay, graphics and you know the rest. Not to say that style isn't useful, just that, from what I've read, it's a bad way to capture what turned this simple but solid shooter into something much more than the sum of it's parts.
To illustrate this, let's take a look at everyone's favorite weapon: the stake gun. Primary fire launches huge wooden stakes that can nail enemies to any floor, wall or ceiling. Secondary fire launches a grenade, either exploding on contact with a direct hit or bouncing around before exploding. Nothing too unusual here, maybe an odd pairing of weapons but that's not all. As a stake reaches a downward arc it bursts into flames, boosting the damage enough to instantly gib most enemies. Pin an airborne grenade with a stake and you send this otherwise short range projectile hurtling forward like a rocket, exploding on contact with the first thing it hits. Impressive, but just for fun, let's say you grabbed the Rage power-up which alters each weapon in different ways. Now your stakes will always be on fire and travel faster in a flat trajectory, while the grenade explosions set the area ablaze.
One weapon! The stake gun is so mighty that description alone should convince you to try Painkiller.
In case it's not there's also the Black Tarot cards. These provide you with bonuses like slowing down time, doubling your reload speed and moving faster. Use all three together and you'll be impaling entire swarms of enemies before they hit the ground. Cards are unlocked by completing unique objectives and, once unlocked, are added to your collection where you can play them indefinitely or swap them out for better cards. A particularly memorable level has you exclusively using the stake gun throughout a medieval "Town" ravaged by plague and fire, chock full of zombies and haunted by flying witches riding flaming broomsticks. No kidding.
To pay to play these cards you'll have to collect gold. A fun and profitable way is to juggle the many ragdolls you'll have lying around. Hit them continuously with the Painkiller's secondary fire to keep them in midair and gold baubles will clink to the floor. This is even easier if they're nailed to the floor. While in the air, gib them with the Painkiller's primary fire or an explosive to maximize the payout. An even more fun and profitable way involves finding holy items that are hidden in each level. They're not only extremely hard to find but often difficult to reach, at first seeming impossible to get to but truly satisfying when figured out. Luckily you can search for these at your leisure, as whenever you complete a level the entirety of it opens up for easy backtracking.
Unlocking enough cards will also unlock the hardest difficulty mode, Trauma, where you'll take more damage, receive less ammo, bodies won't leave souls to collect and quicksaves are disabled. Also you'll unlock two previously unavailable levels, which allows you to unlock two more tarot cards and an alternate ending.
I'm running out of steam so I'll just say that I dove right back in to Painkiller after I finished it and there's not many games I could say that about.