People need to stop going on about how Cassiopeia's identity was somehow changed. It wasn't.

Her playstyle pre-rework was the exact same "land Q -> unload on them with E" pattern that she has now. She maxed E in almost every lane she was in. The only time she would max Q was when she was in a lane where she could not possibly trade with her lane opponent either because of their damage or their range, so Q max would let you waveclear safely and build up to your lategame.

They did nerf her poison damage, though this is lessened by the damage amp on her E. The reason they nerfed the AP scaling on her Q and her mana costs in general was because prior to the rework, Cassiopeia had the same issue that we see with Azir now where she was a lane bully who scaled into a tremendously powerful lategame. That was not balanced and they could not allow her to be a strong champion while keeping that power.

The basic point of her rework was to buff her to the point where she was actually a good champion, but make sure that she wasn't a good lane bully because then she would be overpowered like we've seen with Azir. That's the purpose of her passive: they took damage away from her and put it back in a way that scales in a rather consistent way over time so that she could have strong lategame damage without having a strong earlygame. Her mana costs were nerfed for the same reason: so that she wouldn't be an effective lane bully since she'd need a heavy mana item (and she always built a mana item pre rework, it's not like that changed at all).

So basically, overall her rework did not change her playstyle. It took some power off of her ability to harass with her Q because she wouldn't be able to be a balanced champion with that kind of power, and then buffed her kit generally so that she was good overall now that she existed in a state where she could actually be balanced. She was never this magical "poison mage" idea that people seem to have in the sense that she was never a champion who relied on heavy damage-over-time effects, and her E was always the core of her kit. She was always an AP DPS champion with an incredible lategame, and the way she actually plays fights did not change at all.

One of the biggest complaints I see is the general idea that "she was a poison mage and now she's an ap marksman", but her playstyle was always to land Q and then spam E while kiting pre rework and that's her playstyle now.

TL;DR Stop complaining about Cassiopeia "losing her identity" because the way she plays fights didn't change at all and the way her general gameplay works only changed in the sense that they reduced her lane bully potential in order to make her a champion who could actually be balanced without being overpowered.

EDIT: If you're going to downvote me, tell me why I'm wrong. If all you're going to say is "well her dots are weaker", then the answer is that yes, they're weaker because they let her bully lanes incredibly hard and that meant they couldn't buff her without making her overpowered.