Introduction and Items
I don't play a ton of Cass, but I a had a build up, and I wanted to replace it with a better one. I might say more about how to play Cass later if I get better and feel like I have more to say.
The meta these days
Lately, people usually only run one AP champ. What this means is that people tend to stack health and neglect MR. What this means is that rushing the "-60 MR" build above will have you doing true damage to most of the enemy team mid-to-late game, unless they start buying MR. MPen marks (and optionally, quints) push this even farther.
Even when you're a bit underfed, a solid ult and combo can do a good chunk of damage and win teamfights. In addition, this build isn't really a glass cannon build, while doing great damage against everyone who doesn't stack tons of MR. And in the one-AP meta, forcing them to build a lot of MR can win you the game.
Twin Fang does the most DPS (due to the refresh), and it's how you last-hit creeps. (Q is too unreliable, timing-wise, with its damage trickle in between minion hits). Level Twin Fang first.
Miasma is great for zoning and has CC. It's also the best way to check bushes, and just an all-around neat skill.
Noxious Blast is nice and spammable. It lets you Twin Fang things a lot even if Miasma misses. You can spam it even though you have to be choosier about Miasma.
Petrifying Gaze is how you win teamfights. If the enemy team groups up like a bunch of scrubs, you win that teamfight, usually. It doesn't take long to figure out who knows how to play against Cass and who doesn't. You can do very badly early and turn it around rapidly if they lack anti-Cassiopeia positioning skills. Either get three of them to face-check you, or wait for them to pile on someone, and then ult their faces. Flash might be necessary in some cases. I usually don't mind dying immediately after a flash-ult, because by then I'll have gotten off W, two Qs, and a few Es as well, and the rest is clean-up.
MPen marks and quints for obvious reasons. (Or move quints. Cassiopeia is a semi-good kiter, especially with melee support)
Flat armor and flat MR make laning phase go better. Late-game, you need to either be protected, or make dazzling sacrifice plays anyway. You really need to trade with confidence earlier on, though.
Pros / Cons
I'm just going to list the pros.
Basically, you can be not that good and still do ok a lot of the time. Just don't feed too hard early. Save your ult for when they've already committed, and you can either win the fight outright or set them up to get cleaned up afterward.
When there's just one AP champ, no one builds MR, and you don't need Void Staff. Just build -60 MR itemization, do true damage, and get health and resists as a bonus. An underfed Cass who does true damage still wins teamfights with her ult. If you live long enough to ult two carries in the face, you win.
Ok, I'll list one con. This build isn't good if you have too much magic damage on your team. If a 2-3 AP meta takes off, you'll want deathcap/rylai's/void staff, or something. w/e.
AP mid isn't my favorite role, but Cassiopeia is fun and functional. I seem to be slightly assist-heavy, but it's hilarious when people don't know how to avoid her ult.