As Kassadin got reworked in patch 4.4 I decided to make a guide about my item choices. I've tested it out and like it a lot. Basically you step away from the assassin style and go for a more Ryze style of build.

This is my favourite build. As I said, very Ryze like. This will make you tanky, but it'll also give you tons of damage. You'll get around 700 AP from this build. One thing it is lacking is magic penetration, so if the enemy team has a lot of MR, swap the Rabadon for a Void Staff. The reason Rabadon is here instead of Void Staff, is because, unlike Ryze, Kassadin scales off AP all of his abilities other than his ultimately.

I like maxing E because it has a lower cooldown, it gives you better wave clear and a slow. Q and E has the same base damage and the same scaling, you can ultimately max either. If you prefer the shield over wave clear and the slow, max Q.

Early Game
Just farm, you're not going to out trade anyone any time soon. Wait for the jungler and last hit. When you hit level 6 you should start to think about roaming, but try to push out the lane if you can.

Mid Game
Here you should roam, help lanes and secure objectives like towers and dragon.

Late Game
Remember that his damage is not the same as before, it is a lot less so you can't one-shot someone unless you're ahead. Stick with your team, get objectives and push towers down. Remember to keep vision up on key points.

Teamfights
Since you are tankier than usual you can dive into the enemy team more easily and get away. Try to wait for the enemy team to use their ultimates and CC abilities before you go though, you're not a tank. Go for the AP or AD carry is possible, if you can't, peel for your AD carry.

Kassadin overall:

Less damage
Can build tanky and get away with it and still have damage now
Less risk if you're new to him with this item build, it is forgiving