Kassadin's Stats and Kit (why he is weak)
Stats
Health: 433 (+78 per level) Mana: 230 (+70 per level) Attack Damage: 52.3 (+3.9 per level) Attack Speed: 0.640 (+3.7% per level) Movement Speed: 340 Health Regen: 6.95 (+0.5 per level) Mana Regen: 6.9 (+0.6 per level) Armor: 18 (+3.2 per level) Magic Resist: 30 (+0 per level)
Kit
Void Stone Kassadin takes 15% reduced magic damage and ignores unit collision. Null Sphere Cost: 70/75/80/85/90 Mana Range: 650 Kassadin fires an orb of void energy at a target, dealing damage and interrupting channels. The excess energy forms around himself, granting a temporary shield that absorbs magic damage. Kassadin fires an orb of void energy at a target enemy, dealing 80/105/130/155/180 (+70% Ability Power) magic damage and interrupting channels.The excess energy forms around himself, granting a shield that absorbs 40/70/100/130/160 (+30% Ability Power) magic damage for 1.5 seconds. Nether Blade Cost: No Cost Range: Self Passive: Kassadin's basic attacks deal bonus magic damage. Active: Kassadin's next basic attack deals significant bonus magic damage and restores Mana. Passive: Kassadin's basic attacks draw energy from the void, dealing 20 (+10% Ability Power) bonus magic damage. Active: Kassadin charges his Nether Blade, causing his next basic attack to deal 40/65/90/115/140 (+60% Ability Power) bonus magic damage and restore 4/5/6/7/8% of his missing Mana (increases to 20/25/30/35/40% against champions). Force Pulse Cost: 80 Mana Range: 400 Kassadin draws energy from spells cast in his vicinity. Upon charging up, Kassadin can use Force Pulse to damage and slow enemies in a cone in front of him. Kassadin draws energy from spells cast in his vicinity, gaining a charge whenever a spell is cast near him.Upon reaching 6 charges, Kassadin can use Force Pulse to deal 80/105/130/155/180 (+70% Ability Power) magic damage and slow enemies by 50/60/70/80/90% for 1 second in a cone in front of him. Riftwalk Cost: 75 Mana Range: 700 Kassadin teleports to a nearby location dealing damage to nearby enemy units. Multiple Riftwalks in a short period of time cost additional Mana but also deal additional damage. Kassadin teleports to a nearby location dealing 80/100/120 (+0) magic damage to surrounding enemy units.Each subsequent Riftwalk within the next 20 seconds doubles the Mana cost and deals an additional 40/50/60 (+0) magic damage per stack, stacks up to 4 times.
Kassadin has half his original mobility, 2/3 the power, and is now a more of a risk champion to play.
Now lets look at a more played mid champions stats:
Ahri 
Stats
Health: 380 (+80 per level) Mana: 250 (+50 per level) Attack Damage: 50 (+3 per level) Attack Speed: 0.668 (+2% per level) Movement Speed: 330 Health Regen: 5.5 (+0.6 per level) Mana Regen: 7 (+0.6 per level) Armor: 15 (+3.5 per level) Magic Resist: 30 (+0 per level)
Kit
Essence Theft Gains a charge of Essence Theft whenever a spell hits an enemy (max: 3 charges per spell). Upon reaching 9 charges, Ahri's next spell heals her whenever it hits an enemy. Orb of Deception Cost: 55/60/65/70/75 Mana Range: 880 Ahri sends out and pulls back her orb, dealing magic damage on the way out and true damage on the way back. Deals 40/65/90/115/140 (+35% Ability Power) magic damage on the way out, and 40/65/90/115/140 (+35% Ability Power) true damage on the way back. Fox-Fire Cost: 50 Mana Range: 800 Ahri releases three fox-fires, that lock onto and attack nearby enemies. Releases three fox-fires that lock on to nearby enemies (prioritizes Champions) dealing 40/65/90/115/140 (+40% Ability Power) magic damage.Enemies hit with multiple fox-fires take 30% damage from each additional fox-fire beyond the first, for a maximum of 0 damage to a single enemy. Charm Cost: 85 Mana Range: 975 Ahri blows a kiss that damages and charms an enemy it encounters, causing them to walk harmlessly towards her. Ahri deals additional damage to recently charmed enemies. Blows a kiss dealing 60/90/120/150/180 (+35% Ability Power) magic damage and charms an enemy causing them to walk harmlessly towards Ahri for 1/1.25/1.5/1.75/2 second(s).Enemies hit by Charm take 20% more magic damage from Ahri for 6 seconds. Orb of Deception's true damage is also increased by this effect. Spirit Rush Cost: 100 Mana Range: 450 Ahri dashes forward and fires essence bolts, damaging 3 nearby enemies (prioritizes Champions). Spirit Rush can be cast up to three times before going on cooldown. Nimbly dashes forward firing 3 essence bolts at nearby enemies (prioritizes Champions) dealing 70/110/150 (+30% Ability Power) magic damage. Can be cast up to three times within 10 seconds before going on cooldown.
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(Max Levels)
HP = 1740 - 1759
MP = 1100 - 1420
AT = 101 - 118.6
AS = 0.895 - 1.043
MS = 330 - 340
HR = 15.7 - 15.45
MR = 17.2 - 17.1
AR = 74.5 - 72.4
MR = 30 - 30
So Kassadin beats Ahri in his Health, Mana, Attack, Attack Speed, and movement speed but doesn't gain much from it.
Kassadin's passive is amazing. Until you look at Ahri's and think that her's has more potential.
Kassadin's Q deals a good amount of damage; dealing at most 180 damage +70% of the current ability power. But Ahri's Q deals at most 140 (+ 35% of ability power) damage but if hit twice it deals 280 (+ 70% ability power). Thats way more than Kassadin's Q with more of the risk. If Kassadin did Q and got a shield it would take only 160 (+ 30% ability power) of the damage leaving a whole 120 (+ another 35% (and the initial 5%) of ability power). So that mana shield only helped me to stay alive until her next attack.
Kassadin's W is and I must say his only redeeming ability other than the passive. You deal at most 140 (+60% ability power) and gain back up to 8% mana (or 40% against chamions) for no cost. But Ahri again can cast her W to do auto attacks on enemy champions dealing up to 140 damage also (with only 40% ability power), but gains a bonus 30% damage if an enemy takes multiple hits from her W.
Kassadin's E doesn't even need its restriction anymore. It does nothing now. It only deals up to 180 (+70% ability power) damage after 6 spells have been used around or by Kassadin. He only slows an enemy by 90% for 1 second so it really isn't as useful. And Ahri, on the other hand, can charm her enemies towards her for up to 2 seconds and deals 180 (+35% ability power) damage. She also gains a 20% gain in magic damage towards the charmed enemy for up to 6 seconds. Her Q is also applied.
Kassadin's R is where they hit him hard. Kassadin only deals 120 damage (+ however much you have in mana). The time between each rift is 2 seconds and he only gains a bonus of up to 60 (+ another amount of however much mana you have) to a total of only 200 if you have the amount to use the ability and can. While Ahri's R deals a hefty amount of 150 (+30 ability power) damage per use and can be used 3 times so her total is 450 (+30 ability power x3). That is a lot more than what Kassadin can ever do.
Now if Kassadin had a silence back on his Q for up to a 1 second silence period, then her power output could be halted so Kassadin could do something again. When Riot first got rid of Kassadin's silence, he was still fine but after the 2nd and 3rd nerfs, he isn't playable anymore. Just give Kassadin back his silence. I can somewhat understand Talon and Le Blanc but Kassadin has nothing now.