I did not read Quest for Karn so I'm probably, uh, not going to explain this very well.
From what I understand, though, I don't think the quest itself was a gimmick. Looking for Karn seems like a reasonable plan, honestly. Even if he's the reason all this bad stuff was happening in the first place because he forgot to wipe his shoes before he created a plane. Surely he'd be able to help, right? And then they find him all corruptified - and like I said, I didn't read QFK - and so my mind would immediately jump to "Hey, wasn't there that one Elf lady who makes people immune to phyresis or whatever? Let's go get her!". I don't know if this was feasible, but it SOUNDS reasonable.
Instead, I guess Venser ripped out his heart and somehow Karn got his spark? Yea, that sounds like a gimmick in a sea of otherwise reasonable actions and reactions. OOH, VENSER MUST SACRIFICE HIMSELF FOR HIS ONE TIME MENTOR. TRAGIC. I guess it was a good thing no one really liked him anyway.
I mean, I see a gimmick when I'm reading something, and there is a plain, logical solution right in front of the character's collective faces. And they do something dramatic or stupid instead, for IMPACT. Or SHOCK VALUE. Or maybe even just STYLE POINTS.
As for New Phyrexia, I thought it was pretty obvious where Mirrodin was heading even BEFORE Scars came out. The oil is what Corrupted memnarch in the first place, yeah? Mirrodin was always, inevitably, heading towards becoming New Phyrexia. The Mirran retcon- I mean, resistance, was cute. But the real setting of Scars block is this terrifying new Phyrexian order that is attempting to backwards engineer plane-spanning technology. Like, I wish they had just STARTED there and then slowly revealed that this used to be Mirrodin. That would have been cool.
Yea, I like New Phyrexia as a setting. Tough to write for a plane full of villains though. Unless you're Raven.
The gimmick vs. plot hook thing is actually pretty simple. A gimmick starts something and then never delivers on anything of value. Nothing happens. For instance: Venser give Karn his heart, and then Mirrodin just... becomes New Phyrexia anyways. But at least Karn is alive? By the same token, Avacyn's disappearance. She vanished, nobody knows why, and then bad stuff starts to happen. Then more bad stuff happens, and by sheer circumstance, she is freed, shows back up, and bad stuff stops happening. Then everything is back to normal, with the added bonus of Wolvir.
I mean, it's awfully convenient someone as selfish and spiteful as Lilliana had a score to settle with the demon that ended up imprisoning Avacyn, otherwise Innistrad was doomed. (What was Sorin doing again?)
A plot hook starts something, and moves the characters into the plot. For instance: Frodo inherits a ring from his uncle, and because of the nature of the ring, is forced to flee for his life and go on a grand adventure he never expected to be a part of. It would have been a gimmick if the ring just... turned him invisible and then he was subsequently attacked by weird guys in cloaks for no reason. And then destroyed the ring because it was "evil" and that was that.
Anyway, hope this clears things up. O_o Sometimes I get carried away. -_-