The front page is ordered by

(number of upvotes) / (time in hours+1)^1.6 * (a lot of penalties)

, where 1.6 is a number that changes from time to time, so it may be different now, but something between 1.5 and 2.

There are a lot of penalties, some are automatic, some are added when the users flag the post, some are added by the mods.