Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
BE SURE TO GET A SOLO LANE (or if you are a good lasthitter, get some awesome farm)
"Kassadin is one of the most powerful heroes in LoL and is one that is often overlooked."
That's true and you can use that to your advantage and suprise enemies with your "port in, 2 nuke and leave" attacks which are great for initiating a team fight against unexperienced players or teams that dont have too much stuns or other disables.
My reason to go for Nether blade at 2nd lvl is that if you want to harass as much as possible then you need some mana regen and the passive of that skill provides you with it.
The 3 most important stats for Kassadin are: Ability Power, Magic Penetration and Cooldown Reduction. You can adapt your items to that if you prefer other items then i stated in this build.