ok, i tivo'd this episode in hidef and just watched the cloud vs. eko part in slow-motion. in regular speed, you could see flashes of what looked like lightning in the dark cloud.
however, on replay, it was a bunch of flashes of eko's life.
they showed an image of his brother at current and and younger, an image of himself as a child, an image of the man he shot, a few other things i couldn't really tell but looked like other women (other villagers?), and then a crucifix at the end.
so...
[theory time]
perhaps, the cloud is just the person's soul made tangible.
eko faced his demons in real life, and thats why he wasn't afraid of the cloud, at all.
john locke faced most of his demons and has since come to embrace the island and what it has brought, and thats why he wasn't afraid of the cloud/monster when they faced off either time, last season.
characters like charlie, however, are still terrified of themselves, and thus have nothing but fear for the cloud.
... so, perhaps, the island is like a pergatory of sorts, forcing you to face what you are.[/theory time]
