Main thing to me that post-metal bands have in common is the album artwork, imagery and influence from non-metal genres such as
Drone music and shoegazing. Imagery doesn't seem to be as dark in bands tagged as post-metal as other genres, for example,
Jesu has quite a few songs that could be interpreted as
Negative or positive, for example, "Silver" mentions people walking away, and a new beginning, and the line "you could tell me anything, but not what the future brings" could be taken either way because life could get better or worse. Or at least that's how I look at the lyrical theme of that song anyway.