How can Final Countdown be exploited?
After some research I can provide some insight. To have a One Turn Kill with Final Countdown, one needs to be able to recycle the effect of Pyro Clock of Destiny indefinitely. I found two ways to archive this, both of which are not possible with the current list of banned cards (and possibly will never be able to be playable again), but at least the second variant was playable for a time.

With Makyura the Destroyer

When Makyura is sent to the graveyard, trap cards can be activated from the hand. This video shows how powerful this effect can be exploited. Makyura was banned right when forbidden cards were first introduced, as he allows many other exploits. Basic idea here: Draw your entire deck (easy enough these days, e.g. take variation of Exodia/Dark World deck just like in the video), in the process discard Makyura, play Final Countdown and make some other preparations. Now: 


Play Pyro Clock of Destiny from hand. 
Play The Transmigration Prophecy from hand to add the clock and a Golden Bamboo Sword from your graveyard into your empty deck.
Use another bamboo sword to draw these two cards.
Use another transmigration prophecy to return the other prophecy and the bamboo sword to the empty deck, draw them with the other bamboo sword.
After repeating step 4 two extra times, your two Royal Magical Library on the field have caught up.
Use transmigration prophecy to return the other prophecy and the bamboo sword to the empty deck, draw them with the libraries.
Play the clock.
Use the other transmigration prophecy to add the clock and the prophecy to your empty deck.
Go to step 3.


With Return from the Different Dimension

Return from the Different Dimension lets you summon all of your removed from play monsters. This was always a card that could turn a duel completely around. In the distant past, you needed a working D.D. deck or a chaos deck to make good use of it but since nowadays basically any monster can go to any position on the game table, it is too easy to get a lot of powerful monsters in one turn. For the OTK, we again recycle the Pyro Clock of Destiny indefinitely, this time it is just much more sophisticated. The precise steps are described here, I will give a simplifying explanation. Again we draw our entire deck, get a Black Luster Soldier Envoy of the Beginning and a libraries on the field, as well as three Junk Collector with the use of a Monster Reborn (or whatever adequate replacement is available) in combination with Inferno Reckless Summon. The Junk Collector enables you to activate a normal trap card effect by removing him from the field and the card from your hand. Now the gist of things is this:


Use Junk Collector to activate the clock, The Transmigration Prophecy and Return from the Different Dimension as needed.
Use Primal Seed to return the trap cards to your hand.
Use Transmigration Prophecy and Magical Stone Excavation to get Primal Seed or drawer cards back to hand or them or Excavation back to deck to draw them with drawer cards (like upstart goblin) and the library.
Use Return to reset the Junk Collector monsters.


If carefully done, this can win the duel in one turn.

Alternatives

The power from Pyro Clock of destiny is very impressive. Alternatives I have seen work with Mask of Darkness and flipping a lot. In combination with slowing the duel via cards like Battle Fader and friends, using Pyro Clock of Destiny 5 times a duel seems reasonable. But drawing the entire deck doesn't work well with waiting for Final Countdown, so the easiest way to weaken a Final Countdown deck is to limit the key card. Through personally I don't see that it did that much damage unlimited.