• The humor hits for me. It doesn't for some people. I especially like how the author uses the flow and rhythm of language.
• The structure of the book, in my view, is a stream-of-consciousness vortex circling around an existential crisis, gradually getting deeper and darker. Putting together the connections of these seemingly randomly presented events is interesting.
• The tonal shift of the book, especially its acceleration towards the end, is masterfully done. It starts out as goofy dark humor, and pulls out the humor and increases the 'dark' until it's just unsettling. Still has that bitter irony and absurdism, but it's no longer funny.
• One of my favorite book series when I was a little kid was Louis Sachar's "Sideways Stories from Wayside School", and Catch-22 uses the exact same gimmick for its chapters (each focusing on a character). Similar sense of humor, too.
• I'm a big fan of M*A*S*H, and Catch-22 has similar themes.
• I've read it about 5 times, and each time I read it, I see it from a new angle. To the point where I see interpretations of the story that I've never been able to find online from anyone else. (Does anyone know why Yossarian fell in love with the Chaplain at first sight?)
• Nothing has captured the feelings of frustration and anxiety more than Arfy crowding Yossarian in the bombardier turret and blissfully mishearing everything Yossarian says.

That being said, I enjoyed it most when I was an 18-22 year old guy, and I think you have to have that attitude to really appreciate it. You need that anti-authoritarian streak and a desire to read stuff that has a cynical edge to it, while also being old enough to appreciate the existential issues at its core. I haven't been able to finish a re-read of it as I've gotten older - I've lost the appetite for how obsessed with mortality it is. I also think it helps if you've read a lot, I appreciated how different it was from other books.