Her old passive was only meaningful if you built 
and 
right off the bat (And I literally mean like... five minutes in) in order to avoid mana problems. New Cassiopeia still has mana problems and I would still build those two, but I'm not building them to make use of my passive, I'm building them to eliminate a problem with my mana.
I REALLY shouldn't have to build two high cost items to make use of the passive on a champion. That just plain old doesn't make sense. The other issue with her old passive is that unless you were capable of casting constantly with 40% CDR and an opponent that let you hit them, you would quickly be draining youself of mana to keep the passive up for more than a second. It's the same issue with Mordekaiser's shield: You have plenty of it in laning phase, but if you go without damaging creeps for one second, poof, it's gone. Then when someone comes to gank, you don't know where your shield is, and with Cassiopeia you stumble on mana or cooldowns and get ganked. The passive of a champion shouldn't try to push players to drain themselves of resources just to use a LITTLE less of that resource.
Scrolling down through the thread I was linked, I see someone ranting about how her poison isn't "poison" Well, neither is Teemo's poison, or Elise's poison, or Gangplanks poison, or Singed's poison, or Twitches poison. Poison in league isn't like poison in reality. The champions of league of legends would probably laugh at the efforts of twenty female black widow spiders biting them. Us? We would resign ourselves to our doom. I'm sure you all want to be DOT master mages capable of melting a target and grinning as half the debuffs on the boss are from your DOT spells. But this isn't world of warcraft and we're not playing affliction warlocks. So were not going to be able to riffle the numpad and watch as debuff after debuff stacks on the target.
Skipping to the end, I'm seeing tales where several people have done VERY good as Cassiopeia. And people are just sitting there hamming them about it. I think that the VERY LARGE issue is that people are wearing four sets of Yi brand nostalgiavision goggles. You're having a change brought to a champion that VERY FEW people use to bring them into a state where people can actually use her, and you don't like it. This is the same issue with Twitches ult getting a RENAME. yes, people were upset about Twitches ult being RENAMED and NOT CHANGED IN ANY WAY. That and this are both cases of a very vocal minority trying to revert something that they don't like, yet everybody else either doesn't care or likes it. And it's happened many, many times before.
new champ 
bulf nice