Let's go through the poem in chronological order to consider each mention of death:
Because I could not stop for Death / He kindly stopped for me - Explicit: Death is personified as a polite gentleman who makes time for the busy speaker. Implicit: One does not determine when and how one dies, but rather death makes the decision for one.
He knew no haste - Explicit: Death is in no rush because he is never late. This is the benefit of always being able to determine when things happen. Implicit: Death happens when it will, never too soon or too late.
And I had put away / My labor and my leisure too, / For His Civility - Explicit: To return his polite stopping for the speaker, the speaker puts away her distractions. Implicit: When death arrives, everything that has been keeping us so busy stops.
We passed the School [...] the Fields of Gazing Grain [...] the Setting Sun - Explicit: Death and the speaker are slowly traveling down a road. Implicit: Death can be in any and all of these places. There is no place to which death does not have access.
He passed us - / The Dews drew quivering and chill - Explicit: Death has a cold and unsettling effect on the things he passes. Implicit: The idea of death scares people and stops things from their normal routines.
We paused before a House that seemed / A Swelling of the Ground - Explicit: The speaker and death stop at a small, low house. Implicit: The speaker and death stop at the speaker's grave to deposit her body before continuing on their journey.
I first surmised the Horses' Heads / Were toward Eternity - Explicit: The speaker's journey will be eternal. Implicit: The speaker has reacher eternal life, or death.