Um... most people didn't like the rework because it took her entire playstyle and changed it. Her Q used to be her primary laning tool, having high poke damage, and now it's... a way to set up her E. It does nothing. It's terrible. People complained both because she was weak and because she was a different champion. Going from a long-range skillshot DoT poke champ to a closer range targeted ability spammer are about as different as roles can get, and the people who liked "Poke with poison" Cas were pissed.
Other people, myself included, just thought that putting ALL of a character's strength in a single ability was a terrible idea. ESPECIALLY since it's a targeted ability with no real counterplay. New, reworked Cass is the new Ryze. She zones you for a week by spamming her E and bullies you out of lane while scaling insanely into lategame. They gave her a WORSE playstyle than before in regards to counterplay. She can really only ever be hugely OP or hugely worthless now... since she's 90% based around a single ability with no other options, gimmicks or playstyles to fall back on. If her E is strong, she's going to be a bully and a monster, and if it's nerfed into being weaker, she won't be good for anything.
People didn't bitch because she was weak. Old cass wasn't really a powerhouse. People bitched about the rework because they ripped away her identity and playstyle and gave her a new one that not everyone liked.