Why I don't think Kassadin is op.

"Well too bad Kassadin main your tears are delicious", "LoL another whiner complaining about your freelo champ getting nerfed", "Well riot clearly thinks he is, so enjoy your nerfs bronzie"

Well I will say upfront that I'm not ranked, and have no particular interest in ranked play; that said, if you want to say something like the above, I've done your job for you, so you don't need to feel the need to post just tell me that. :-)

Now onto the topic at hand:

Because of his ultimate, Kass was often referred to as the most mobile champion in the game - however, this is somewhat of a misleading title. Other assassin champions (and even some non true assassins like Ahri) can, in specific situations match or even beat Kassadin's mobility in terms of applicability: Akali's ultimate is on a shorter cooldown, does not have energy requirements, and resets on a kill giving her excellent chase potential. Fizz has 2 seperate dashes on separate cooldowns that grant him untargetability or extra damage respectively. Zed can perform multiple dashes in short succession, combined with using his dash points as seperate remote directions of attack. So, rather than saying Kassadin is the most mobile champion, it would be more accurate to say Kassadin is the champion with the most immediately accessable, general purpose mobility, and it comes at the much higher cost of quadratic mana scaling.

People when referring to Kassadin often complain about mobility coupled with high burst - however this statement is patently innacurate; Kassadin does not have high burst. Kassadin has the lowest base damages of any assassin with the possible exception of Master Yi (who is an auto attack focused champion, rather than a burst caster) and Evelynn (who is also in an overnerfed state, though the on-hits on her E do look promising). His damage steroid (the passive on his W) basically functions as a weaker Nashor's tooth (already an extremely situational item) and is extremely limited in application compared the steroids of many other champions like Shaco, LeBlanc, Zed and Katarina which are much more damaging up front, and burst oriented; Kassadin's requires him to hit with auto attacks to be effective, which plays against his current pattern, since he no longer has the intrinsic attack speed bonus' of his former incarnation. Simply put, compared to every other assassin, his damage is at best sub-par.

To re-emphasize just low low Kassadin's damage actually is consider this - Leona and Nami both out-damage kassadin, and they are a tank/support and support/mage, respectively.

If (in my view) Kassadin is so underpowered as an assassin, why has he been going up in popularity and apparent strength? This change exists largely as a result of the huge mana regen nerfs for this season - previously, mages were able to overcome their mana hurdles by building a , allowing them to spam most of their abilities as soon as they were off cooldown, letting them both farm and harass at range with little consequence. Since the changes to this season with regen being largely useless (again, your opinion may differ here), combined with Kassadin's emphasis on building flat mana, and mana sustain from his W, Kassadin is able to outlast the more powerful ranged abilities of his mage opponents and farm long enough to gain some level of combat relevence. So it's not that Kassadin has gotten stronger, but rather that Mages have been severely weakened as of this season.

"But Kassadin is super strong! OP Even!" Well, if you took all of Kassadin's abilities (sans the ult) and you looked at them, compared to any other assassin champion, you would say hands down that he would not be worth using over any other assassin champion. Even with the reliability of his abilities, his damage is lower than every other assassin in the game and lower than most mages. With long cooldowns, and stack requirements, combined with being a melee range champion, Kassadin's kit is loaded with constraints - indeed, he has neither the damage of an assassin, nor the utility of a mage, but rather the high mana costs of a mage and the squishyness of both. However, where Kassadin excels is in his application of damage - his mobility allowed him to get into position in order to finish off escaping, or severely weakened opponents while being able to escape retaliation from his opponents. This means that Kassadin is strong, not as a DPS assassin (like Master Yi) or burst assassin (like Talon) but rather as an opportunist, finishing off weakened opponents. He is strong on capatilizing on weakness and escaping, which gives him the appearance of omnipresence and makes his damage look scarier than it is.

So what was the point of all that long winded nonsense?

tl:dr Basically, I don't think Kassadin is OP, or even particularly great; he has low damage for an assassin and low utility for a mage. His kit is riddled with long cooldowns and stack requirements, his power budget is taken up by things that don't really help him do his job (that is killing people) due to his theme as an "anti-mage" (something which Veigar beats him at), and mobility costs higher than any other assassin in the game.

This ult nerf will probably ruin him, since his strength was in applicability of damage, rather than doing high damage - but if this allows Riot to finally let him do damage again, maybe it won't be so bad.