Operation "Zombie Bunker" DLC adds 3 nice single/coop maps, with one of them beign completely new. They are themed around military and thus it means that they heavily use hitscan monsters. Which means that there will be difficulty spikes, yes.
Stalingrad, which was called Leningrad in Battle Out Of Hell, came back and shines in all the prettier look. With some small details. Aside from final arena statue not getting bombed. In this level you will have to deal with hordes of common soldier demons, coming both from Nazi and Soviet forces, as demons make no difference. It somehow felt less cheap. No sniping soldiers in windows that appear behind you. Generals (or whichever title is correct in this case) don't call hard-to-avoid airstrike at you, instead shooting projectile directly at you. Got some details, like airplanes bombing some streets.
It has different less interesting battle music though. But don't worry, corrupted soviet hymn wasn't omitted fully, you will see. So, it's mostly positive changes.
Bunker is totally new level developed for HD and is quite unique. It doesn't feel budget and it could fit into Painkiller. just imagine being in the dark nazi fortress, at the night, as light projectors check the skies. Atmoshperic, with nice soundtrack. And it playes on "Nazi Zombies" trope too! I said zombies, not demons, they are different.
And it has a lot of them. Over 600+ enemies on a map. The biggest one in Painkiller HD. Though, it also means that it's kinda on dragged side. Not so many points of interests. But at least it still doesn't feel as bad as Painkiller Redemption or Recurring Evil. Since these zombies die easily and all.
Oh, yes, and at this point you will be introduced to the new weapon. A freaking mace. Animation is lovely, making this weapon feel heavy and powerful, that's for sure, as you swing it left and right, charging it up and then throwing a mine at somebody. Fits to zombie carnage too.
Military Base is another map ported from original Painkiller. It's quite huge one, you will have to bunnyhop around a lot. And well, it's also not all that nice with hitscan enemies in open space, you know. And it also feels mostly copy-pasted, not exactly upgrade in the looks. But it also adds new small details without taking any, so it's another positive port, even if level itself isn't all that great. Blame original Painkiller on that.
You will also notice DLC description talking somethnig about two new rocket launchers. It means enviromental trap. Like, in Military Base rocket launching machine has button on it which you can press. No, they don't aim at enemies, so it's kinda useless.
Tarot card is something about armor regenerating after 10 seconds. Or something. Not sure about details, I didn't pay attention during gameplay and didn't feel any effect it does.
Yea, and I guess you will want that multiplayer map for survival. Not sure.