In Well's book, when the green comet hits the atmosphere a green fog envelops everyone and they fall asleep (creating chaos as bus drivers et al also fall asleep - anybody ever forced to watch that terrible evangelistic movie Thief In The Night in the 70s? or the almost as terrible Left Behind? Yeah, sounds like that...). Anyway, the green fog changes the air and when everybody wakes up they have a utopian mindset - a world war immediately ceases, the main character's difficult romance is easily sorted out, etc.
So, that would be the kind of contrivance used by a game company seeking to reboot their game setting (Time of Troubles with Forgotten Realms from 1e to 2e, Grehawk Wars as well, probably others...didn't really keep track) but I don't want to have the "everyone falls asleeep for three hours and wakes up to a whole new world" happen on my world.
But it does have to affect it. Apart from the ecological and geographical changes that would come, what else? What if it NEVER hits the atmosphere (if, for example, the PCs decide they want to try to stop it and they succeed?). If some cultures would worship the comet - let's say as the spurned lover of the sun - then how would they respond if the comet went away? Or, if it seemed to head toward the sun? Hmmm....
Like a thousand other things, this'll be percolating in the background (I don't think in straight lines, so this blog is the perfect place for me to throw out my disorganized thoughts - with the hope of one day organizing them...).