There it is!
1. SHOW ME WHAT YOU GOT
Throughout the game, you, the Penitent One, will meet giant black skulls with faces of molten gold who will give some backstory and exposition about the "penances" (stages and bosses) you will be facing. I'm not much of a Rick & Morty fan, but almost every reviewer or LPer makes this reference.
2. I'm not sure that's how bells work
One of the locations is the Great Bell of Jondo, which is so massive it's basically a titanic, valley-sized metal bowl dug into the ground.
3. The saintlier you are, the bigger your corpse gets
Many of the game's screen-spanning bosses are Saints and the Church hierarchy, most of whom are already dead, but reanimated through the grace of the Holy Miracle.
4. You will feel VERY guilty for missing that jump
One of the game's distinctive mechanics is the Pillars of Guilt, Dark Souls-like specters of your past self generated every time you die. The twist here is that, if you don't go back to pick them up, your Fervor (MP gauge) is permanently decreased, and gets further diminished if you die again before collecting your Guilt. If you're feeling lazy, you can also get rid of the penalty by paying a fee to a Confessor (presumably, the Penitent One goes, "Forgive me, Father, for I missed a jump by one pixel and fell in a bed of spikes," I guess.)
5. Good giant baby, bad giant baby
There's a lot of babies in this game! Jocinero is a miraculous giant baby that lives within a painting, and can give you significant powerups as you finish his quest. Esposito is a baby sanctified by the Church (therefore, giant, see question 3) who lurks in the background of a boss fight, and will grab you and instakill you if you get close.