Contractual Boss Immunity: Most large bosses are immune to knockback.
There are two bosses that instead get a fullscreen title card just before they appear.
Boss Subtitles: All bosses get a subtitle/namecard on the lower-right area of the screen, with the exception of one (Soul Warrior).
Battle Theme Music: "Decisive Battle" is the standard boss theme, "Dream Battle" is used for Warrior Dream bosses (save for No Eyes, who sings her own song), minibosses usually use the heavier Variable Mixes of their dungeon themes, and major bosses have unique themes.
The only exception is the True Final Boss battle against The Radiance, presumably because, as a Physical God and leader of a tribe who excelled in mucking about with dreams, she knows exactly how to truly kill you in one. You don't have to collect your shade (and thus don't lose your money) and the three Fragile charms don't break, allowing you to use them without fear (which especially comes in handy in Godhome).
Dying while in the dream world has far fewer consequences than dying in the normal world.
Although every Boss Rush battle at Godhome culminates in a fight against a brand new boss, merely encountering them is enough to make them appear at the Hall of Gods, allowing you to practice against them at your leisure.
But this is especially obvious to those with asymmetry in their sprites, like the Hollow Knight's scar and arm, Grimm's cloak, and Zote's horns.
Ambidextrous Sprite: All bosses flip sideways depending on your position, thus switching their handedness.
the Mantis Lords, Dung Defender, Warrior Dreams, etc.) bleed the usual white blood of bugs instead.
Since a lot of them are infected, they spurt out orange blood when hit.
Tropes that collectively apply to a majority of bosses listed here.