The masteries aren't about personal preference, it's that almost all the offensive masteries are useless for Kassadin. You get more damage, more cooldown reduction, and more mana with the utility tree. Telling people that it's just what you like to do, and that it's your opinion isn't something that should be in a guide.

In the boots section you never mention the Sorcerer's Shoes for magic penetration, his most common choice.

If your build focuses on mana, then Archangel's shouldn't be something optional, like where you say to get that or banshee's.

Mejai's late game is a pretty bad idea all around, if you plan on getting it at all, it should be early on, after you get one or two kills, and you know you're going to do well.

This guide really needs a lot of work, and you don't really explain how to play Kassadin, so much as just talking about what items he could use.

This guide needs a lot of work.