Cassiopeia is most dominant in the lane phase of the game, so it is most important that you do exceptionally well early with her. Her play style is fast action micro with extreme prediction of your opponent.
Cass is a traditional spec of 9 points in Offense and 21 in Utility to maximize cool-down reduction and help her with mana. I put only 1 point in utility mastery because you will not always be given blue, and some games baron is not even gotten.
Her runes are very custom to her play style. Mark of insight for magic pen. Greater seal of replenishment in the yellows, and Greater glyph of Replenishment in the blue. I take both mana regen here because, Cass' whole game is dependent on how well she does in her lane phase, this will allow you to have nearly unlimited mana to harass, and farm efficiently. Finally Greater quintessence of Fortitude for early game cause she only starts with 4 bubs.
Very simple. Q for long range harass, and being able to quickly close the gap if you land your spells. E second to compliment your combo. W was not very impressive but great for poising in combination with your ult.
Cass is a wicked micro character that takes a lot of practice to master. I feel Cass should be played with extreme aggressiveness. Trying to gain control of what ever situation she is in. Also she is extremely good at manipulating the bush.