Ooooh boy. /Sam Beckett voice.
There are three things working against Catcher, starting with what
@overgeeked mentioned: Americans like to be pissy about what they're assigned to do in school.
And yes, Holden Caulfield is whiny, especially when you look at the fact he's from a VERY upper-middle class family: Apartment in NYC, private schools, summers in Maine. BUT, he's also dealing with unresolved trauma surrounding his brother's death; his mom is as well, and as a result has shut out her other kids; his brother is implied to have seen some of the worst combat in World War II; Holden's crush was sexually assaulted by her stepdad; it's implied Holden and another student were both sexually assaulted at school; the popular kid gets away with date-r*pe in the coach's car: there's an undercurrent of mental health concerns that wouldn't be out of place in a novel written today. And this kid: moneyed, educated, bright--he should be doing GREAT despite all of that. But he's not. Because underneath it all, there's the harsh reality that the systems that do exist to support this kid have failed on every possible front
Frankly, the subject matter and point of view would be celebrated as "raw and real"--in a novel written today.
That said, a lot of the high school students who read "Catcher" today can't relate to the point-of-view of Holden's socioeconomic status, and a lot of teachers (in my limited experience of teaching the novel for 23 years) don't address the trauma Holden is dealing with*, rather choosing to focus on the angle of "He's an anti-hero, a teen (like you, my students!) who speaks truth to power." Not that he's a broken kid for whom life froze five years earlier. When I teach it, I (try) to bring in one of our school counselors or social workers to talk about the systems that failed then and that exist today.
It's also a victim of its own success. It was one of the first novels from that voice to "break big," and (as it does) exposure led to fatigue (see Drizzt or Lord of the Rings for examples related to why we're all here). And along the way there have been countless imitators, iterations, and other angsty teens dual-wielding scimitars. (Wait, ignore that last one.)
It's also great for teaching voice and unreliable narrator, which, for my sophomores, dovetails well into developing critical thinking. Holden sets up right away that he's not going to tell us the whole story, so we (my students and I) need to turn on our "horse hockey" detectors right out of the gate. It gives students a chance to really analyze what IS true, what is PROBABLY true, and what is CLEARLY him trying to be cool. It can be a lot of fun to sift through his self-aggrandizing and read what's being said under the surface. That's when many of my students really start to relate and understand the character.
And since Catcher has spawned so many imitators, those are skills that are just as useful in reading other works. And literally everything on the internet.
I came to "Catcher" later; we didn't read it in my high school (rather, "Franny and Zooey"), so I didn't read it from the same age-POV as Holden; I was twenty-two or twenty-three when I finally read it, so I had some more maturity on me (my co workers are laughing at me using "maturity" and "me" in the same sentence). And when I read it the first time...yeah, I thought he was a whiny kid. But as I looked at Holden and I looked at my students, I saw a lot of the same concerns presenting themselves. So, honestly, yeah. It's one of the most "Real" books I've had the opportunity to teach, and I do think it's a "good" novel.
*That's not an indictment. Time, curricular dictates, comfort with mental health issues, how it was taught to them: all of these can affect how and why a teacher approaches the novel. I have two education degrees and a counseling degree. The mental health angle comes naturally.