The Famous Five follows the tried and true formula of having a group of mischievous but basically moral kids go on some kind of adventure with no adult supervision but everything works out splendidly in the end.
Seriously, I first started looking at this genre as an adult with the Rover Boys, which first came out in 1899 and it wasn't original then. I'm not sure how far back the genre goes but I can argue that the Coral Island could be considered an example and it was from 1858. The Adventures of Tom Sawyer came out in 1876. So Five on a Treasure Island wasn't breaking new ground in 1942.
Plotwise, Blyton didn't bring anything new to the table. A group of kids go on holiday and thwart some bad guys in order to get a treasure. I will give props to Blyton for making it very clear that George's family has clear legal ownership of the lost gold.
So, is there anything that seemed special about the Famous Five for me? Actually, yes. In almost all the Stratemeyer Syndicate books I've read, the characters are all virtually interchangeable. With the Famous Five, while the characterization is still flat, they are still distinct. Not exactly groundbreaking but it does give the series an edge over other many kiddy adventure books I have read.
George weren't special mention since she isn't just a tomboy but a girl who actively wants to be a boy. Many modern critics have interpreted her as transgender. I am very sure that's not what Blyton intended but, if she did, good for her.
I am not planning on going out of my way to read the rest of the Famous Five books. They definitely scream product of their time, with all the classism that implies. However, I can see why they stood out in the pack.