@feld Interesting, I'd say networking is something newcomers can quite quickly grasp, at least thanks to Wireshark and ability to use pretty much any language including say Python, plus just poke around.
That said I'm probably quite biased as I started with network software (pretty fun to write clients/bots/…).

Meanwhile I'd say system is the worst, lot of documentation though but also lot of NDAs and barely any tools with a decent learning curve specially when you go low-level.