Jaina is LITERALLY a win button. I have never seen someone activate Jaina and get the benefits of her effect and not lose. Games with Control Mage ran like this.
Are you at turn 9 and above?
No? Well, keep trying!
If yes, is Jaina in your hand? If not, just wait a bit longer. If yes, congratulations! You win!
The fact that she spawns an Elemental when she's played is the cherry on top of bullshit. She got really insane when Rastakhan rolled around and Odd Mage had Voodoo Doll AND Janalai ready to say the big 'ol Fuck You to anyone who isn't playing Control.
That's quite a statement, gul dan was staple in every warlock strategy and saw play even in zoolock, tyrion got equality nerfed, valeera got kingsbane nerfed, rexxar had 60% win rate when played post ww and anduin had raza nerfed and took away the meta for 4 months.
Jaina on the other hand saw extremely mediocre play except in the last expansion cycle.
Rexxar's power wildly varied depending on what was in the beast pool at the time (and Hunter's ability to make it to the late game). When the hero was launched he was crap.
I don't think any of the DKs in of themselves were too strong, they all felt really balanced to me (especially compared to the weapons and the quests). Just having that hero card which generated so much value on it's own feels really strong whenever it's played.
"I have never seen someone activate Jaina and get the benefits of her effect and not lose."
That is such hyperbole. Most combo decks beat it. Most aggro decks beat it. Did your sad midrange/control deck get countered? I'm sorry, but I remember even my control warrior and warlock decks outvaluing it and winning. You have to be careful with your trading and keep track of their cards.
Let's be honest HS always looked up to MtG while trying to be casual friendly. Sadly they tried to copy in a way the planeswalkers style of card but came up with a very nefarious card. Hero cards are fun but all of them should be 10 mana no exceptions
Sometimes it's best to take it slow and not play anything to give her a ping target, Control Mage has very few actual threats. Then try to set up for your own big swing-turn that they can't easily board-wipe.
As a heavy user of control mage with DK (but only when control mage was good) i do think it is overpowered. At one point it felt straight up broken as the meta had only paladins and even shamans. But after a lot of priests joined the meta (and more came after release of broken shit like catrina), i think Jaina DK is overpowered as a single card, but the deck doesn't do well this time, even with jaina on 9.