Cassiopeia had two main issues before her rework: her passive forced a power curve in a way that didn't feel good for anyone, and she felt off thematically. Her passive offered power in such a way that she simply didn't have a passive in the early game but became an unstoppable god late, no matter what she or her opponents did; her current passive means that late game she has an extra item slot, allowing her to still get that extra edge in itemization while still depending on her ability to farm and get a gold lead.
Her thematics are a little less simple; basically, pre-rework, she didn't feel like a poison mage. For me and a lot of others, this was primarily because no matter how strong her DoTs were, their primary purpose was never the damage, but the steroid that they offered Cassiopeia. By shifting the poison bonus from a reset on Cassie to damage on her opponent, the poison feels more like you're inflicting vulnerability and weakness on your opponents rather than just buffing yourself.