My opinions:
QNX is great, but RAM-hungry. Sure, you shouldn't need to swap, but it happens... (What're the chances of *sshd* being killed in an OOM situation?)
FreeBSD starts to feel a bit cramped in 2.5GB, if you're making a proper effort to keep /usr/src around (and conducting source upgrades, etc). That said, the recent 4.x-branch releases are quite usable (tracking the current release of your major branch is a good idea in FreeBSD-land); 5.x will show some major benefits for SMP, but all that advancement does start to make it look a bit big/complex for a set-and-forget uniprocessor box.
NetBSD and OpenBSD are 'lithe' in comparison; I'd call OpenBSD for situations where the machine will be its own firewall, and NetBSD for other "experimental" tinkering.
I know little about the state of the major Linux distributions, but it's becoming clear that RedHat should be "tracked" like FreeBSD; the *point* of new releases is in fixing the bugs of the old ones, and the EOLing tells you you *won't* be getting security advisories/patches - if their current offerings are too bloated, it's time to upgrade your hardware (as Windows users must), or switch to another *NIXalike distribution (Slack, *BSD, Debian...) with a different engineering philosophy. No use crying over spilled milk, though you're welcome to cry over the quality of distributions still making allowance for lighter systems.
Mac OS X is much the same way; 10.1, .2, and .3 are closer to 'snapshot' releases even than the Windows-of-the-year model. This release model isn't so bad -- upgrading, say, FreeBSD from 4.7 to 4.8 has results little different from applying a FixPak to OS/2 or a Service Pack to Windows -- but the difference is that FreeBSD (and RHAT?) let users move up for free, while Apple's shrinkwrap philosophy amounts to a $100/year subscription fee. (Ask me how satisfied I am with SBBOD bugs in 10.1 -- and 10.3 coming soon enough that it's better to leave my cousin without a laptop than have him cough up for soon-obsolete Jaguar.)
