People obsess over the compiler flags (and there was a short time when everything else seemed to be x86-32bit and Gentoo was one of the very few distros where you could compile everything for amd64) but the real advantage of Portage is the USE flags; where you can turn off major things like X support, or disable IPv4 entirely, etc. Nothing else I've ever encountered allows that customizability.

And that's been much more important to me than cflags, which I set once and ignore, ever since trying to install a command-line MP3 player on an ancient RedHat brought in an entire X install.