Flash - You always need flash to escape from a position, to catch a kill or to engage a teamfight.

Ignite - It comes very handy to get kills early since you have a strong early game.

Teleport - You can roam, follow your midlane opponent or to come to lane early.

You need damage and you need to be tanky to survive eaiser in the teamfights. So combination of these items give you big amount of HP and damage.

You need to max Q since is your main tool for damage. Second max is your ability E, if you catch your opponent with it you can right click on them and they will dash in your way and you can follow up with Q for some poke and dmg. W is good for vision.

Arcane comet is the best for your runes since your W has very long range and also you can hit your E quite easily. Perfect timing is follow up and you also need it later for zhonyas.