Some striking similarities are;
1) Band names are mind boggling
Take Super Junior, 2NE1, DBSK, or even After School. Maybe one group will come up with the name Kimchii next time. Nama sayur tu..LOL
2) They look insanely good looking.
Well, I can say most of them are. From the kawaii ultra cute look to the sassy or dashing look. Those perfect hair, perfect nose. Just don't be too naive to think it is all au' naturale. Cosmetic surgery is very big especially in Korea. Having your nose done is just like walking to a hair saloon and getting your hair washed.
3) They sound generic.
After a song or two, you can definitely hum to almost any songs. After all pop songs are meant to be catchy and predictable in a way. One thing that I see a bit different between K-Pop and J-Pop, K-Pop tend to sound more mature than the latter. My personal opinion anyway, people will disagree as well.
Nonetheless, on top of all this sugar coating and to make everything nice...what matters at the end of the day is how marketable they are.
And surprisingly they do! They sell very well.
Currently am hooked with this group.