I think the most eye-opening were the Little House on the Prairie books.
That made me think of something else that is not exactly on this re-reading topic--or rather, it is not only on that, but also--but is not far off and does include. I read several of the Little House books, but started with 'On the Banks of Plum Creek'. I had no idea these were celebrated children's books. And although 'On the Banks of Plum Creek' is the only one I didn't finish, it's also the only one that transported me into a singular world which exists nowhere else. And this leads to not wanting to re-read things or re-see certain things if you can't get that same exact feeling again. This by no means always applies, but sometimes I feel that if I re-experience a work a second time, it will ruin my memory of it, because that has indeed happened. It's just like going back to some places that transformed your whole worldview, and when you return they aren't the same place, and you're disappointed.
Other oddities are having read 'kidnapped' and 'treasure island' when I was 7, but just reading them straight through and paying no attention to what they were about, just to show off I could read 'hard books' for my age (I found out later this was no great feat.) These I think I will reread to see if I remember even a single image from them.
Another phenomenon was trying to read 'Vanity Fair' several times and never finishing it as a teen-ager. I still haven't finished it. I got rid of all this feebleness as an adult and took 3 1/2 years to get through 'Tristram Shandy', which I found maddening, but thrilling.