A few months ago I read the rest of the Linux filesystem code, wrote a few notes, and forgot about it. This write-up is going to therefore be a bit vague.
nfs is centred on proc.c, which implements all the tedium of RPC, and then the rest is pretty much wrappers around this.
msdos - all I have is a note "Thoroughly bored by namei.c". I can't tell if this particular file was boring, or if this was the last file in the directory, and I was bored by that point. There's a little bit of caching around the FAT layer because FAT is basically a poor structure for large file systems, but I don't remember much else being interesting about it.
hpfs The implementation is all in one file, but it's pretty readable nonetheless, perhaps because of its limited scope - it's read-only, which makes life a lot easier. It appears to be based on an old paper, rather than solid knowledge - a bit of a reverse engineering exercise by the looks of it! The code is interestingly written in a top-down style (a style discouraged by C's scoping) - I think this helps with its readability.
ext2 The famous, venerable ext2! Compared to ext, there are a bunch of little, neat changes. You can put a small amount of data inside the inode itself, for "fast links". Block sizes are now variable. There's preallocation code, so that blocks can be allocated together for efficiency. There are a certain number of blocks per group, which hold together blocks and inodes, presumably also for locality. Bitmaps for inodes and blocks are back - I guess this is to allow support for preallocation.
On top of the basic storage level, there's a direectory entry cache for files with short names - which is what you end up needing if directories aren't structured for fast search. Rename locking is improved, which is nice as the whole area looks like a pile of race conditions waiting to happen. There are plans for ACLs, but they aren't implemented. And one of the stranger things it has is automagic upgrade of filesystems from the 0.2b version. Putting that in the fs layer itself seems a bold move to me, but there you go.
... and that's it! File systems are done. Next up: Sound devices, which should make a nice change from the core, standard stuff.