He had everything in his locker to become an one of the all-time great wingers, but somehow he didn't. Technically he was perfect, both feet, shooting, crossing, dribbling, an insane pace... he was an even slicker version of the young Ronaldo. Such a shame. This is something that Fergie, I think, didn't handle perfectly — it was a bit like Sven-Göran Eriksson putting Scholes on the left because the lesser players were less versatile (he actually said that). When Nani was at his best Fergie shifted him to the left because we also had Valencia (in great form), who could only play on the right. I'm not sure that it would've really made the difference, but I would've loved to see peak and fully confident Nani performing in his best position for a prolonged period of time.
Never understood the hate for him though, although I seem to react differently to inconsistent but insanely performers like Nani, Pogba etc.