Encryption is clearly a core feature of APFS.
APFS brings a much-desired file system feature: snapshots. A snapshot lets you freeze the state of a file system at a particular moment and continue to use and modify that file system while preserving the old data. It does so in a space-efficient fashion where, effectively, changes are tracked and only new data takes up additional space.
APFS claims to be optimized for flash. Flash memory (NAND) is the stuff in your speedy SSD. SSDs mimic the block interface of conventional hard drives, but the underlying technology is completely different. In particular, while magnetic media can read or write sectors arbitrarily, flash erases large chunks (blocks) and reads and writes smaller chunks (pages). The management is done by what's called the flash translation layer (FTL), software that makes blocks and pages appear more like a hard drive.
APFS includes TRIM support. TRIM is a command in the ATA protocol that allows a file system to indicate to an SSD (specifically to its FTL) that some space has been freed. SSDs require significant free space and perform better when there's more of it;
The problem with TRIM is that it's only useful when there's free space: it's something of a benchmark special. Once your disk is mostly full (as mine are in my laptop and phone basically at all times) TRIM doesn't do anything for you.
APFS also focuses on latency: Apple's number one goal is to avoid the beach ball of doom. APFS addresses this with I/O QoS (quality of service) to prioritize accesses that are immediately visible to the user over background activity that doesn't have the same time-constraints.
It's a shame that APFS lacks checksums for user data and doesn't provide for data redundancy. Data integrity should be job one for a file system, and I believe that's true for a watch or phone as much as it is for a server.
