Hard to say
I think when you see it in person it creates this powerful sense of illusion, somehow - because of how realistically these near-life sized bodies are painted, the painting creates a vivid illusion of bodies floating upwards.
It's supposed to be an image of salvation, of the bodies of the dead rising on the day of judgement, but it's really a quite sinister, morbid image. I like that apocalyptic tone it has, the grim colours.
It's also a bit kitschy, as Leighton's stuff always is, the marbled/tanned muscularity of the man. I say it's painted "realistically" but of course it's not a realistic image, both in the sense that it's a mythical/magical scene and in that it's highly mannered.
BUt it's that collision of realism and fantasy that makes it so weirdly powerful, I think - as an image.
If its intended to have an emotional effect (and it probably is), a poignance or something, it completely fails in my eyes, because of that mannered, uncanny quality.
These are all groping explanations to explain a visceral response. Probably too there's something satisfying about the way he's positioned the figures in the foreground and background, a mathematically satisfying balance, within the circular (tondo?) frame, presumably derived - as is the anatomical depiction of the bodies - from Michelangelo and Raphael.