Ninja, way better than every other class.

[Allyrion]

It wasn't really op at the time, it just looked like it was.

Poisons giving 20% damage? Doesn't matter because the damage numbers were balanced around that existing.
TP costs low? Yeah they were a little low, but the adjustment brought them pretty much in line and it wasn't a huge adjustment
Damage? I believe Aeolian Edge got a 5% nerf a little time after release. So one ability was very slightly too strong. So OP!

The attraction for me was the ninjutsu requiring individual mudra input. I thought they could do some cool stuff with a system like that, and so far they have (though they could do more).

The combinations are very cool.
The system allows NIN to have 13 Job abilities vs the 5 everyone else gets.
It's really a good way to include an interesting mechanic, provide a wider variety of abilities and still reduce button bloat.
That's one of the main reasons I play NIN, in addition to the magic-melee hybrid as well as being able to glamour swords.
Still waiting for SAM though.

They're doing a similar design with things like the AST cards and the new stances as well, I think.