This is pretty much the problem. Cassiopeia was my first champ I bought, so I can see where you're coming from. Cassiopeia is a super mana hungry champion, but I don't think giving her EXTREMELY high mana sustain and spellvamp for free is the way to go about it. Her old Deadly Cadence passive was much better, in terms of being less obnoxious as hell, because it contained her mana problems without making her "might as well be an energy/resourceless champ" in lane phase at least. The E mana restore makes her mana management braindead early and strangely bad late unless you get a tear... It's very awkward.
That said though, the logic here riot is presenting is mind boggling. "We wanted to tone down Cassiopeia's oppressive lane bullying." Okay, so why did they buff her Twin Fang, which was the reason short range mages and melee mids were tearing their hair out before they could get enough items to fully burst her? Her Twin Fang was where most of her oppression is. It warps to being bad for Cassiopeia on the other end of the spectrum, too. Because her E had so much power her high poke damage was unreliable. (Q was a good poke spell, W had good zoning but it was unreliable as poke) Against high range mages Cassiopeia did struggle. Her Q poke was NOT enough to keep up, unfortunately. ( At least for me. I'm at like high silver now and I started playing at the end of season 3 so bear with me, please.) This translates to basically "I shit on your melee mid, or Annie, Ryze, etc. but I'm mediocre against poke/ long range burst mages. (Thank god Wyndra got gutted)
Now, with players being forced into the Twin Fang all in playstyle, the issue is all the more exacerbated. She still shits on melee's/short range mages, and now she can farm it up EASILY against these long range poke mages now because of her health and mana sustain. Rush a catalyst instead of a Tear if you're frightened on getting bursted and then you will be a self-sustaining menace as long as you need to scale up. Honestly, she's still a better lane bully than late game, imo. She's been buffed enough to have a steady 55% win rate.
http://forums.na.leagueoflegends.com/board/showthread.php?p=10780961#10780961
This is an old post with Brackhar, Cassi's original designer, discussing her Deadly Cadence passive. This passive was honestly pretty damn good considering how mana passives are boring from the get go. But, it enhanced her lane bullying without giving her INFINITE mana sustain, and the stacking mechanic added a sense of rhythm to your spell casts. It was pretty engaging for a mana conservation passive, instead of the one on Twin Fang she has now.
http://forums.na.leagueoflegends.com/board/showthread.php?p=4613331#4613331
Brackar had intended to create Cassi as a champ similar to Anivia, although mobile and sustained damage focused rather than slow and bursty. You can see from her old kit these elements. Cassiopeia's Miasma was quite similar to Anivia's Glacial Shroud as a good zoning/waveclearing tool. Also, look at both of these champion's E spells; Twin Fang and Frostbite. **Notice how on their own they deal mediocre damage but with their other abilities they can do a lot? And these other abilities are plenty powerful on their own?**There are other examples, but basically I feel like so many parts of the zoning-control mage/Anti-Anivia are... not the same with Cassiopeia anymore.
I'm going to say a reversion of the rework is in order. And perhaps to never rework her. Cassiopeia's two playstyles as a poison-poke mage/ or all in DPS mage with twin fang appeal to very different sides of her fanbase. If one side HAS to get snubbed, in this case it was the players that played her as a poison-poke mage, than maybe it's better to leave her with her slightly awkward, but EXTREMELY MORE enjoyable and engaging kit she had before, and this can be based on her old and new passives alone.