Not sure if it's because of starting as a web/application developer and but I wish I was introduced earlier to:

- in-process databases (rocksdb, sqlite)

- FoundationDB

- C/C++ and low level programming in general (I wished I learned those instead of js when I was younger)

- State Machines, Actor Model (Orleans Net), Event Sourcing

- Bittorrent for other things than pirating (it looks like it's dying)

- Arduino and similar

- Seastar

- Arrow (ecosystem)