Hello my name on the NA server is Ericles, I love to play mid lane assassins. You may ask: "Why Kassadin, isn't he absolutely useless because of his minuscule ult range?" In fact despite Rito's shortcomings they did give his ult reduced mana cost an AP ratio and buffed his W. Kassadin superbs in countering mages and the infamous Katarina. His kit from bottom to top counters magic damage ability reliant champions. With season six just around the bend, ADCs are ridiculously strong, and the mid game is where the game's fate is decided. Therefore, Kassadin's crazy burst and mid game powerspike would be extremely relevant in deleting enemy carries and carrying the mid game.

Why teleport over ignite?
Teleport offers you all the extra map presence and back door potential you need, and allows you a free back or more in lane. Whereas ignite offers some kill potential. Safety first ;)
Why flash?
That is not even a question. the question is: Is it on F or D?

Start Q to trade/harass/farm this is your main spell to be using pre-6 where you are practically useless with no gap-closer. Ty to farm up for your catalyst and DON'T DIE! Take W at level 2 unless your are against someone like cassiopeia, who spams spells and will need to be peeled away, in which case you would take E. Use your W for mana sustain so you can throw Qs without going OOM. Dont try to kill your laner unless they step within flash combo range within ~30% hp.

Take W against another all melee mid and Q against a melee mid with ranged spells (i.e. Zed). Farm up but look for trades with W AA reset. You can consider maxing W in lane over Q to fight them off ESPECIALLY IF THEY ARE AD.

Take a point in your ult at level 6 and look for roams. You should start maxing E starting at 7 for the extra wave-clear. you can use your ult to move across the map but keep an eye on your stacks. Snowballing bot lane will skyrocket your chances of winning. If you can a coordinated gank with your jungle could turn bot lane from a 2v2 into a 4v2.

You should have ROA and should be planning your next item based on the situation. Buy Zhonya's for teamfight's, Lich bane for assassination, Luden's for wave-clear... (etc). Your goal is to kill their ADC in every fight or keep him shut down, prioritize the enemy's to threat level to YOUR ADC. So probably a riven and a talon would be high n the list.

Don't flame at your team, mute them if you have to. Don't tilt, play smart, play safe. And carry their sorry ***es.