User blog comment:NightbladeX/Kassadin Rework: Thoughts/@comment-19287469-20140326131921

The Kassadin rework turned what used to be a massive power curve into a massive power curve that is incredibly snowball-reliant. He's even more item-reliant than he used to be, as his core build now relies on both mana and AP to deal damage. A Tear into Rod of Ages start is almost crucial if you want to use your Riftwalk, else your mana cost will be too high to sustain more than three in a 12 second window, AND it won't be doing any reasonable amount of damage. In addition to this, those two items mean there's no conceivable way for you to get the five other items that are virtually required on Kassadin- Deathcap, Void Staff, Zhonya's, Lich Bane and boots.

This said, some have said that Kassadin should be built tanky, with emphasis on sustaining your damage. This would involve something like boots, RoA, Frozen Fist, and several other items. Problem is, Kassadin's bases are so ridiculously low, even with a healthy splatter of AP and mana, you won't deal enough damage to give the enemy team a reason to attack you, as well as the previous problem of being incredibly item-reliant.

Tl;dr: Kassadin used to be a champion where you sit and desperately try to survive laning phase, then crush everyone within 2000 units with your insane ratios and 5-second-blink. Now, Kassadin is a heavily item-reliant champion which lends to the nature of making him INCREDIBLY snowball-focused. A Kassadin who gets ahead is unstoppable, but one who's behind isn't worth your autoattacks.