What happened to Cassiopeia?
First of all, Cassiopeia was the very first champion I ever picked up. I still remember how her unique design caught my attention when I was scrolling through the massive champion-roster on your site, I instantly knew that I had to play her. That's how I got sucked into this great game which I'm playing to this very day.
Since then I've become a dedicated Cassiopeia main on EUW with over 900 games on her alone. Indeed, she has become very dear to me which is why I must address the points which I'm about to make. I'd like to talk about the major predicament that she has become as a champion and how she continues to struggle within a merciless meta which condemns her existence.
Many players including me feel that Cassiopeia herself hasn't aged very well over the years, her mechanics have been left untouched and her kit hardly changed at all. The result is a champion with heavy problems at positioning herself in a meta dominated by sheer mobility, hard cc and instant burst-damage.
Cassiopeia herself is a short-ranged, squishy, dps-mage - properties which turn her into a huge liability for her own team later on. Whereas other squishies have some sort of backup-mechanic to secure their survival (burst or/and mobility to quickly get out), Cassiopeia doesn't have one.
Her laning-phase is quite the tricky pony aswell. You either snowball early on by getting first blood, thus enabling you to acquire the vision and mana-regeneration you desperately need, or you risk falling behind.
Should she die during the first 5 minutes of the game, you might aswell consider your lane lost. It's quite easy to snowball against Cassiopeia, a champion who is incredibly dependent on items to make up for several of her handicaps.
Now even if you manage to secure yourself a dominating role, things quickly change during the later teamfights. Suddenly you become much more passive due to the constant threat of getting jumped upon while trying to hide behind your team. Too bad your buddies only have eyes for their adc.
Speaking of adcs, if she's supposed to play like one, how come she can't have any decent spellvamp of her own? Meanwhile adcs gets to make use of inherently broken lifesteal-items which fill up their health after clearing a single minion wave, for absolutely no cost. They don't have to rely on mana just to keep on attacking while Cass clearly does.
Okay, so she has a two second stun on her ultimate but that's meant for dis-/counter-engaging. Don't expect to make any plays with it, you'll be lucky enough if your team even follows up otherwise leaving you behind with up to 5 enemy champions which you can impossibly melt down on your own.
She's obviously not suited for solo q play and requires a great deal of coordination within your team to make full use of her abilities, yet competitive teams refuse to pick her. Can you guess why?
She doesn't have any other means of peeling for herself except for blowing her Summoner's spells. Experienced players can easily detect the casting-animation of Petrifying Gaze and simply turn around to avoid the stun at all, others simply jump/flash out of there.
Speaking of her ultimate, I still can't understand how this ability has been left untouched after all those years. Its range is pitiful and comes with an awful delay that can be interrupted at any time. Yes, you still hear the scream but the ult won't go off. Sure, you might say that may be necessary for counterplay but if you're already in range of her ultimate, you deserve to be punished.
The biggest issue however has to be the actual hitbox of her ultimate - I'd like to share some screenshot-evidence on that case: http://postimg.org/image/u0ktkgwc7
As we can see, enemies like Lee Sin can easily jump on Cassiopeia thus avoiding getting hit by Petrifying Gaze entirely. Sure, you might say that's intended due to the animation which makes Cass appear to move forward but I'd like to share another example: http://postimg.org/image/6rps9sfs5/
How come something like this is allowed to exist? Lee is clearly getting damaged despite staying BEHIND the source of Luxanna's laser. Why can't Cassiopeia recieve the same treatment?
Of course many of you will try to bring up her amazing damage-potential but what use is all this if you can't stay alive long enough? You instantly get focused down and in worse cases your ultimate even gets interrupted by hard cc and death itself. Even being knocked into the air negates Petrifying Gaze entirely.
So what's the verdict? To make it short, Cassiopeia is an incredibly situational champion with delayed skillshots who lacks any reliable mobility or inherent defenses who constantly has to deal with a meta which literally punishes your existence.
There's a reason why Cassiopeia hasn't seen any play in a competitive environment since a long time, and that's for a good reason. High risk, high reward? There's no reward in playing this champion anymore.

let him
against