toru hagihara was promoted midway through sotn's development, and then iga was made the new director...after he became director, they basically restarted the game i guess.
Is that fact, or myth/legend created from fan speculation? There is no doubt that Iga had a great deal to do with the success of SotN, and it was an amazing game. But he tends to get ALL the credit for it, like the idea above that they restarted the game halfway through after he became director. Its hard to believe that they would scrap a game halfway through development, start over from scratch, but give the director credit to the guy who's version was scrapped halfway through.
If i start stealing aspects from other games and combine them into one game, it's the same as the sprite reusing that IGA is doing in most of his games. If you want to make the difference, then try and make something original and not stealing aspects and ideas from other games that other publicers have come up with.... 
One of the key factors that made Symphony successful, and also all of its GBA/DS offspring that Iga is famous for, was that they "borrow" heavily from Super Metroid, ideas from another game series, that another publisher came up with...
Even the idea of breathing new life into a dying series by putting the Metroidvania format onto a handheld... Iga didn't innovate that either. The Circle of the Moon guys came up with that idea, and then Iga very successfully copied it when he took over the series.
I would say that most of Iga's success has come from successfully incorporating great ideas from other developers into his Castlevania games.