Default system uses less memory than Ubuntu (~50MB last time I checked which means something on .5, 1GB RAM instances). I could swear memory consumption in general has fallen down, but haven't done any measurements.
I don't have to jump all over the web to figure out how something works - I just go to handbook. Which means one gets tendency to study topics instead of copy-paste snippets from around.
Since base system (maybe things were different before) does not abruptly change or pull the rug under your feet, combined with handbook studies, this has the effect of compounding knowledge that'll be effective in years to come.
Firewall (pf) is a thing of beauty. I've barely scratched the surface of it.
Base/userland abstraction split, as well as consistency throughout the system helps one maintain a solid mental map of the system. After a short while, you just know where stuff is, how to configure it, and where its defaults are even when you're in completely new territories.
Bonus: You can really go the distance without compiling anything, but if you're keen on maintaining a low memory profile by dropping various features from your packages, this is a great new build system I use to have all my pkgs up to date - https://github.com/jrmarino/synth