One reason I dislike the trope is that it's a way for a very lazy writer to take a very lazy shortcut. "Why is my character the Savior? What does he do to deserve it, to make him special, to justify that everybody is going to place their hopes in him? Oh, I don't know, building character is hard work, I'll just stamp his ass with a crown tattoo and that'll sort it out. Plus any actually insurmountable problem that appears in his way can be shot down with a Deus Ex Machina because hey, he's the Chosen One, things like that are supposed to happen around him." See Secret of Nimh 2 for an example.
The other reason to dislike this trope? As someone else said in this thread, it's childish. That is to say, exactly the fantasy of a child. Kids dream that they're not just one of the homogenous masses sitting in the classroom, in reality, they're THE CHOSEN ONE! Not because they're achieved anything great yet or have any special talent for world-saving (being kids, that's perfectly normal); it's just because they're born special or the Lost Princess or some other invention. They'll do great things and save the world; mom and dad will be proud and their classmates jealous! I think the reason we get tired of this cliché when growing up, though, is that we come to realize that anything worth doing should take hard work, dedication, talent, sweat and blood, and the courage to face the fact that those efforts could still result in failure. To an adult who has to take risks and work hard to achieve anything worthwhile, the idea that somebody is going to become The Last Best Hope For Mankind just because they opened the right cereal box or were born that way is...annoying and unrealistic.
Asita said:
Po from Kung Fu Panda. The guy had an intimate knowledge (read: obsessive fanboy tier knowledge) of what he was chosen for but lacked the requisite physical ability. Everyone was flabbergasted at the idea, because he seemed like such an unlikely hero. And he was good enough once he got trained, but for a good portion of the final battle the villain wasn't taking him seriously, and the very thing that led the others to be similarly dismissive of him ended up negating the villain's trump card...at which point the guy fully lost his cool and got his tail handed to him as a result. It's like the Dan (Street Fighter) psyche out in movie form. His greatest strengths are that nobody takes him seriously and mostly underestimate him as a result, and then they start panicking and lose their groove when it looks like he might win.
Oooh, I like your choice
And Po's role as Chosen One actually does make sense. Shifu is upset with Oogway's choice because he thinks Po's just some random panda who barged in at the right (wrong?) time and got picked as Dragon Warrior for no reason, when he's obviously not built for it. In short, Shifu sees him as what I described in the first paragraphs. Oogway, with greater clarity, sees a guy who loves Kung Fu with such passion that he strapped fireworks to his ass and rocketed fifty feet in the air just to see someone else get chosen as Dragon Warrior -a purity of passion and purpose that Tai Lung (obsessed with power rather than the art of Kung Fu) did NOT have. And the fact that Po survives this treatment is a good sign he has the physical potential to be a Dragon Warrior whatever his shape
Buffy is also a great example, because the Chosen One title also makes sense in that context. It's not just a little tiara with 'You're Special! You'll Win The Day!' on it; it comes pre-packaged with a toolset - the strength to take down vamps - and a mission - to take down vamps. Simple and elegant, and it does not insure success or even that you'll be any good at all, or a hero, as Faith illustrated. In fact the one thing it does insure is that you'll meet an untimely and pointy-toothed death. Or worse. The only way Buffy 'wins' this game in the end is by breaking the trope (but I won't spoil the ending of the last season).