The visual design is awesome, and those guys at people can fly/farm 51 proved it with their other games as well. Same for ambient sound, while the music is a solid bunch of metal tracks. At the opposite of serious sam, each level has its own style, which is even more stunning considering that each level brings an almost perfect atmosphere, and this is why painkiller is so good. It's not just shooting enemies, it's shooting enemies in such an awesome background I can't even describe it. It's like a fairy tale, that's just it.
The engine, well it's havok, awesome physics and graphics, and even now it's doesn't look like it's old.
The storyline is completely empty, but who cares, it's an old fps.
The gameplay is just awesome:
-movement: you can bunnyhop and then change direction without losing your momentum
-weapons: not a big amount of them (which is good here) but really distinct weapons with clear, fun, and new mechanics that brings complete new gameplay with each one of them
-enemies: mostly dumb, some special attacks and "teamplay" with monster interacting with others to create new attacks
-bosses: zelda bosses: really big and funny
-secret areas like any other old fps
And now this is why I think that game is also an old platform game like:
-you have stats for each level (completely different levels, like in sonic you would have some city, then a desert), you have a "hell map" where you can play and replay each level to boost them
-for each level you have special objectives to get a bonus: a card
-you have to find gold (yeah I said it) to use those cards
In short that game is pure fun, no need for useless complicated stories/mechanics/character to protect/objective to capture/bomb to desactivate, it's just pure visual/gameplay fun.