Ra's al Ghul is one of the few major non-white comic book characters, so the opportunity to cast a non-white actor is too good to pass up. Who better than Naveen Andrews? Already a recognizable face from Lost, he could certainly be a believable Ra's al Ghul. Sayid was a conflicted torturer, then a ruthlessly efficient assassin for Benjamin Linus once he got off the island. At only 5'9 he wouldn't be much of a physical threat to Arrow, which goes against the show's current thinking to ratchet up the threat to Oliver. Each villain appears to be more a challenge than the last. Think Deathstroke after Merlyn. Ra's al Ghul needs to be very intimidating. With Andrews the focus would need to be on the intellectual danger he poses. He would certainly be a sadistic menace to Arrow. Especially if he can deduce the Arrow's identity as he famously did Batman's. Or they could emphasize the character's ability with a sword, a way he's challenged the Dark Knight in the past.