So, how do we know the difference? And how do we avoid a hook becoming a gimmick?
For me, I like to look at already-produced works as examples. For this post, I'm going with romantic comedies because they often have strong (and sometimes outlandish) "hooks" to lure movie-goers, and most of us have seen or heard of them, so they're easier to talk about (I also realize my selections are entirely subjective -- movies that I wasn't crazy about might be your all-time favorite, or vice versa)...
HOOKS that worked (for the most part):
* The Holiday - two unlucky-in-love women, on different continents, swap houses for the holiday and find love again. (Cute, sweet, charming. Not perfect, but for me, the hook delivered).
* Letters to Juliet - engaged girl travels to Italy and discovers a letter "to Juliet" written 50 years ago, then sets out on a quest to reunite the woman with her "Romeo." (The trailer looked cheesy, and yes, parts of this movie were, but the romantic in me LOVED the idea).
* While You Were Sleeping - woman falls in love with man, who falls into coma, who has a brother, who falls in love with the woman, "while he's sleeping." (I'll admit - when I first saw this movie trailer, I thought the movie would be totally stupid. But it was so charming, so well-written, that it sucked me in, and became one of my all-time favorites).
*honorable mention goes to Return to Me (David Duchovny, Minnie Driver). The "hook" is downright ridiculous, in its over-the-top/uber-coincidental plot - man loses beloved wife; her heart ends up inside a woman who needs a transplant - who also just so "happens" to fall in love with him a year later. But - the execution of this story is nearly flawless. Witty, smart, heartfelt, charming. One of my all-time favorites.
HOOKS that turned into GIMMICKS, or just didn't work (again, just my opinion, here...).
*Sleepless in Seattle - yes, I know I'm risking blasphemy with this choice, lol - this is a HUGE classic, and I adore Meg Ryan. But - for me - the "hook" (two people "meeting" long-distance through a talk radio show) didn't work, because of the ending. We never got to SEE their relationship truly form, and I felt like the movie just began when they laid eyes on each other and walked into the elevator. It left me hungry, unsatisfied. Good hook, weak execution.
* Runaway Bride - she's a runaway bride and he's the reporter covering the story. The dream team, Gere/Roberts, couldn't save this film for me. The hook was clever, but something about the writing fell flat and felt very contrived.
*Kate and Leopold - time-travel "hook" - gorgeous Hugh Jackman, a 19th century duke, is transported to modern day and falls in love with Meg Ryan. I'm on the fence about this one - I wanted to love it - the cast is tremendous - but again, the execution fell a little flat for me.
Just in viewing those examples, I would say that authenticity and execution were what set the good apart from the bad. You can have a FANTASTIC idea for a novel, but if it's not executed well, it won't matter. If the plot doesn't flow and make good sense, or if the characters are doing things, well, out of character, then the hook doesn't matter. It becomes a gimmick.
On the other end, if the hook itself feels contrived or manipulative or totally unrealistic, it probably won't work - no matter HOW well it's executed ("Return to Me" is the exception for me). But if a hook itself feels very natural, feels somewhat plausible, then yes, I think the reader will be happy to go along with it.
What are some other examples you can think of, regarding hooks and a gimmicks? They don't have to be romantic comedies. I'd love to hear them!