Linux

Any advice from people that already use Gentoo? Especially things that they wish they knew before trying for the first time?

This is more things-about-bootloaders I wished I'd known/remembered when installing more recent systems, but:

• If you want to use GPT partitions on a system with legacy BIOS boot, remember that you will need an additional unformatted small partition at the beginning of the disk (this tidbit of information used to be in the handbook, but has been removed)
• If you're trying to install GRUB on a UEFI system, and it looks like everything should work but the system fails to boot, read the troubleshooting section of the GRUB page in the wiki for information on how to handle defective UEFI implementations.
• Do not try to enable Secure Boot if this is your first rodeo. Get the rest working first.

How much time and effort should I put into fine tuning the global USE flags if my initial goal is to get a system up and running?

Not much, unless there's something you really want to exclude (-systemd, for instance). Most of the time the default USE flags will give you a workable, feature-rich system (and allow you to use more binary packages).

What you need to pay attention to is your choice of profile, which sets your default USE flags. Profiles in the desktop series enable a lot of USE flags. For the most basic command-line system, you're better off picking default/linux/amd64/23.0 (or default/linux/amd64/23.0/split-usr if you want the old FHS setup where /bin and /usr/bin etc. are different directories—if you don't care about this, stick with the default and ignore split-usr). Avoid no-multilib, hardened, and anything marked "(exp)" or "(dev)" unless you have some specific reason for wanting that profile.

With systemd, I enable --nowed that which I wanted to “autostart” (iptables and sshd, for instance). Is there an equally intuitive counterpart with OpenRC?

I think you want the rc-update command, specifically rc-update add [service] default (assuming you want the default runlevel, which you nearly always do). There's a man page.