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Jax was, and still is, the worst champion in the game. I'm sure people will argue me on this, saying that he couldn't be so bad as that, not after his buff. But I contend that no other champion is as unable to use his abilities in a team fight and so every other champion at least contributes more than Jax.
And that is his his major problem: his ultimate and stun are nearly useless in a team fight because one requires a lot of build up hits and the other requires something to attack him (and if he's being attacked in a team fight, he'll certainly also be CC'd, for which he has no answer because he needs Tabi instead of Merc Treads).
Ganking is also incredibly difficult with Jax because he has no natural slow or speed increase and is so vulnerable to CC.
I think his super-nuke that was mentioned is only really possible for a very short time in the game, where Jax is at least level 6 and people are still laning, and even then it is very situational to execute, requiring mistakes by the enemy, and it doesn't do as much damage as a real caster would deal.
Look at the strategy listed above: auto attack creep once, leap, auto-attack, empower. This sounds good on paper, but in reality, it is not so simple to just "auto-attack a creep, then leap" because the enemy champion is not going to be just standing in range of your leap while you hit a creep unless he is about to CC you.