Are they compulsion effects, overriding the facilities of mortals at key moments? Do they influence moments of random chance, weighting the scales of the universe towards a particular outcome in general? Most importantly, what happens when a Sufficiently Powerful mortal declares "**** this noise." and ushers in a new era, the 'Age of You Are ****ing Dead If You Start ****.'?
Encoded into D&D on a bone-deep level are two assumptions; random chance and the actions of certain individuals simply cannot be predicted or accounted for. As long as D&D is played as a game and not as a pre-determined-outcome collaborative storytelling exercise, it is impossible for anyone in the universe to know the outcome of certain events with certainty, because random die rolls and the ability of the characters to make decisions are both out of the purview of the game universe. As such, destiny is, at most, a rough map of the way the future might go.
Don't get me wrong, I like the idea as so presented; in fact, I make it a point to include an active, quasi-anthropomorphic force of destiny in most of my campaign worlds. However, I do so treating it pretty much like any other NPC; I give it rules and limits, decide its goals and how it hopes to see them enacted, and then have it compete with every other character in the universe fairly. Hell, this is D&D; I say give Destiny stats like Pratchett's stories (detailed in Witches Abroad); we absolutely be able to kill fate and take its stuff.