K-pop is taking Western music, combining it with Western concepts, Western production, Western sonic trends, and Western psychological fangirl-baiting to create a popular culture trend based 100% entirely on Western culture. There are no fucking differences. That’s not good or bad, but that’s what it is and don’t let others tell you different.
The only thing Korean about it is that it’s happening in Korea, which means that the competition is tougher — they’re all trying harder than everyone else to create the perfect pop product because they’re culturally perfectionist workaholics who run on two hours sleep.
I don't disagree with anything that author kpopalypse wrote, but my own analysis of the difference between the two pop styles would be this:
K-pop is basically Western pop with the pitch cranked up to cartoon-voice level and the rhythm sped up to a frenetic, cocaine-crazed pace.
Like much of Korean pop culture, K-pop feels like a lighter, cheaper, more effete, and very derivative version of its sturdier, more original Western counterpart. Think about kitchen utensils. If you're from the West and you move to Korea, one of the first things you notice, when you go shopping for utensils to stock your kitchen, is that all the utensils are made of lighter, weaker, cheaper metals and plastics. Everything is more breakable and less substantial. That's K-pop in a nutshell: a lighter, fluffier, substance-free version of Western pop: easy on the brain (or, in my case, harsh on the ears) and completely unmemorable.
This isn't to say that I hate all modern Korean music. I've heard some pop songs that had a hint of substance to them—some singers with rough-edged, powerful voices, who didn't sound like the latest "Bristina" clone. I've heard some modern songs with heavier beats, slower tempos, and a more thoughtful sound overall, and I've appreciated them. But such songs are few and far between in Korea.
This also isn't to say that I necessarily find Western pop superior to K-pop. In general, I despise Western pop almost as much as I do K-pop; it's just that I grew up listening to Western pop, so I'm more used to it. I've never gotten used to modern Korean music.
So, basically, I'd argue that K-pop is the Pikachu-infused bastard child of Western pop: all cheek-lightning and high-pitched voices, possessing even less substance than Western pop does. It's auditory cotton candy—eminently consumable, but there and gone. I wish it would go away, but that's not happening anytime soon.
*Not sure how much feminists will appreciate it, though, given its accusation that "fangirl idiocy" was and is one of the engines driving the entire pop movement. There's also the "erect dick to hop on" comment at the end. I found that comment funny, but that's because I don't have an anti-phallocratic stick up my ass.