Pain & Suffering is a modpack based around a few core ideas- And surprisingly, they aren't "Make the player suffer." They're designed to create an intense, nonstop action, chaos-fest.
1) Anxiety. At no point should you ever feel safe, at least not until late game. You should always be on your toes, scared of what comes might attack you from where you least expect it. Nowhere is safe, and your best bet is to stay moving, fighting monsters, and looting for gear.
2) Randomness. While there are some strategies that you can learn, the loot and enchantments you will end up using will be different for each time you play, and the enemies and ways you die will be varied. It's not unlikely that you'll find a sword that for some reason does 60 damage per hit, only to get 1-tapped from full health by a skeleton 60 blocks away... Despite having a full set of powerful armor.
3) Confusion. Now, the point here is not to confuse the player of "Oh drat, now how the heck do I accomplish this task?" Rather, it's to make them ask: "What is THAT?! What are THOSE!? How did THAT happen?!! Who even killed me?!?! That EXPLODES now?!?! I thought that was SAFE!" You should be constantly finding new freakish abominations to die to, and constantly adjusting yourself to realize that a strategy that worked a few minutes ago might not work every time.
4) Convenience. Mods and config changes are set in place to make your experience less boring, and help you stay in the action. You should never feel slowed down, or tied to a location, or bored of a repetitive task. "Return to base?" Nah, you've got a capsule. "Storage issues?" Nah, AE2. "Smelting?" Nah, Better Furnaces.
All this is to culminate in the goal that there should never, EVER be a dull moment.

