BASH shell, stability, flexibility. native development tools (gcc, perl, etc.). Central repository of programs (not just system programs - I try to get most of my software out of it so I can update EVERYTHING on my system). Freedom of information (flexibility) - if you want to change something about your system, search for it. when you find it, do it. when it's done, it's done.
I've come across a few times when something's happened that I thought I couldn't get out of (WINE went into some kind of debug loop), but all it took was a second to sit back and think and I got it cleared up right away, without comprimising anything else. Other examples: me accidentally killing my own X-Server, starting a program that took WAAY too much processing power
the ONLY time I've had to restart Gentoo was when I was messing around in my system and plugged something in... I got used to the stability and decided to open my case and mess around with the internals with the system up and running.