I guess I am (un)lucky enough to have to deal with multiple OSes - Windows, various Linux and proper Unix - on a daily basis, so can generally pick out the most suitable for a given task/use. OOF runs on Linux, as it fits Linux's niche very well (low cost internet services). However, it also highlights the fundamental flaws in the Linux kernel (in this case made worse due to the architecture of the forum software we use). OOF could run on virtually any OS, but think Linux/Unix will always be the most suitable for it in its current form.
This is the key, IMHO.

Having cut my teeth on embedded systems I know only too well that if your kernel doesn't fit the requirements of your application the whole system will be an inefficient kludge. Furthermore, if your hardware platform is fixed and doesn't allow you to throw RAM and MIPS at it until it works you are really up the creek without a paddle.
Doesn't really matter so much on a general purpose / desktop machine because you
can throw resources at it until it works, the machine is not likely to be heavily loaded for more than a few seconds at a time and there is a wide range performance over which the machine is acceptable.
What surprises me is that it's so common to be using a general purpose operating system with a hugely complex kernel (I'd include all flavours of Windows / Linux / Unix here) in a single-purpose machine such as a web server. Surely a nice, simple RTOS that can implement a web server, PERL, PHP, a database client and a few other bits and pieces would make far more effective use of the machine's resources? It could specialise in processing 1500 byte blocks of RAM and chucking them at the network interfaces pretty much to the exclusion of anything else. Its simplicity would also make it more stable and more resilient to attack, surely?
Anyway, must stop waffling.

Kevin