Every time _I_ have to deal with linux I am pissed by the in my eyes poor quality of the manpages, the incosistency in the system, and the often insane defaults. When I read Linux code I am scared by its often bad style, use of magic numbers, questionable hacks and obfuscation, compared to the clean code we try to use in OpenBSD. And often enough I am scared by a very sloppy dealing with copyright.
A special case are firewalls - over the last, well, it's nearly 3 years, pf developed rapidly to a fairly impressive packet filter, with a lot of surrounding applications to turn it into bigger solutions. I don't see any comparable packet filter in the free world, and I dunno about commercial ones, bout I doubt there is any.
Well, iptables may be able to do most of what pf can do, but it does many things wrong IMHO. And the concept of formulating firewall rules in command line options to some tool is so obviously flawed that everybody is using some frontend, with makes this further confusing and complaictedm and the resulting ruelsets worse, and... well, just compare to the beauty of a well written pf.conf.