Ahri - I feel her e (charm) has way too high of a mana cost that you can't even use it twice without draining yourself completely of your mana. Towards mid/late game it doesn't tend to be a problem, but I definitely feel it's way too high of a cost early on.
If Ahri could spam her charm it would simple be broken. Ahri has been in a great place for months now due to her damage and outplay potential. She is pwerful, but you can counter her charm by keeping a minion between Ahri and yourself and punishing her once shes used her charm and force her to ult or flash as an escape.
Velkoz - He's able to spam completely and his mana costs are all really low. (Can someone explain this one?)
Velkoz has low mobility. Keep moving. Assassins are a great counter, you'll get zoned if you try to fight Vel koz with an immobile mage.
Fizz - When he goes under ground with his E (Trickster?) I believe its unfair that if a katarina q or some target attacks it, he wont deal damage. (Logically speaking, it makes no sense. Just was wondering if this is how it was intended to be.)
I absolutely detest Fizz as a midlane main. My current champion pool doesn't do its greatest against him and I do not seem him with enough frequency to dedicate time into a harder counter. I try zone him with ranged attacks, I'd be lying if I said I understood how he works.
Karma - I feel as if her cool downs/damage output early game is nearly unmatched, but is this just my thoughts?
I do not see her mid too much. Never really an issue when I do. Don't know enough to discuss. I just shoot spells at her. Lol
Zed - A full combo with zed (W>E/Q) and any mid lane/adc squishy (all) are nearly half hp with thunderlords. And since he runs on energy, its hard to compete as he can outlast most mana champions if they drain and try to get him lower.
Zed runs on energy and can outlast you if you fire off spells that aren't effective. I typically shut down Zed with Brand by harrassing with my passive, or Heimer + Zhonya's.
Riot has said a big update is coming for the mages and I'm patiently waiting. I'm sure we'll get a bunch of mana changes. Honestly, I miss Clarity, I used on most my mages and mana potions, oh how I miss them. Clarity allowed me to fire off attacks, and get my mana back to prepare for a counter attack or just continue to move offensively down the lane. Sorry if I went too off topic but mana wise I just wise mana management was easier in the lower levels. I'm truly feeling the effects of losing Clarity and mana potions.