I'm writing a hobby kernel/operating system and one of the things that surprised me was how simple ext2 is. It really isn't much different from fat32 in many ways, just a bit more flexible and designed in a more future-proof way.

Obviously ext2 is a very outdated filesystem by today's standards, but considering ext3 and ext4 are essentially just some extensions slapped onto ext2, I still didn't expect it.

Another thing I found really impressive was just how resilient e2fsck is. During early development of my ext2 code I frequently mangled the file system in a myriad of weird ways, but no matter how much I broke things, e2fsck was almost always able to restore the file system into a reasonable state.

(For the curious, there is some excellent documentation on the ext2 file system at https://www.nongnu.org/ext2-doc/ext2.html that I mostly used for my implementation of it. Sadly no such thing seems to really exist for ext3/ext4).