Out of curiosity (as a player relatively new to dominion) what's to prevent having a dedicated "kassadin killer" in the mid area. A random idea would be nocturne, who could spellshield his silence, permaslow with phage/frozen mallet, and ult to catch him when he tries to run. Obviously the kassadin killer would have to be fairly good to keep up and there aren't tons of champions that could pull it off, but a roamer that focuses on negating kassadin's effect by lurking in bushes and jumping him every time he tries to gank would reduce his power. Another alternative would be leblanc who could chain silence him (engage from bush to prevent him silencing first) to prevent his leaps and silence and even chase him if he runs. I mean, they wouldn't render him completely useless, but...?
My person favorite is playing Fizz and baiting Kassadin into dueling me. Most Kassadins are usually stupid enough to try it, and fail to realize that Fizz outduels them. Usually only works once or twice, but it's still satisfying.
Talon with ignite can jump on Kassadin and instagib him - provided he builds no armor. As soon as he does, this doesn't work anymore.
LeBlanc's 4 seconds of silence + 1 second root help in dealing with him if she catches him. The issue is that she needs a friend with her to help finish off Kassadin. Kass' passive + RoA and BFT make it impossible for LeBlanc to finish off Kassadin before he rift walks away into the next health relic.
Nocturne doesn't work because most Kassadins are going to try and engage him one on one. As soon as Noct engages on him Kass can riftwalk over a wall (and in many cases out of LoS). Global ults like Noct, and Panth can help alleviate some of the pain Kass' mobility provides, but only once every few minutes. Often times Kass can simply riftwalk back out and go back to top while your Panth/Noct are stuck bot. They get back to the action much more slowly than Kass does.
The issue with all of these "counters" is that if Kassadin has half a brain, he won't fall prey to them.