Heh. Eric pretty much posted the entire list. Everyone else is taking a chance. FreeBSD is the kind of animal where one plans ahead and matches the hardware to the best drivers.
Nothing to stop a thread listing great combos with a few pros and cons. Get the ball rolling and if it is popular there is no reason it wouldn't be made a sticky. I think on one level the mods don't want to spit out a component list, as it might be construed as endorsement of vendors etc. Cyber's list is incredibly thorough, even if a bit overwhelming for some.
To me there is one head and shoulders above no brainer. x10sl7-f, e3 1230v3, seasonic psu (450w+), micron or hynix ecc ram, wd reds, nice open case. Great value and stability saves the cost of the hba. Everyone wants to add a "personal touch" But it is often just a source of pain.
I do think there is room for a "logic" based list. One that would target cost, performance, stability, size, flexibility, power, etc. All those targets result in minor tweaks. All require a fair bit of knowledge to execute proper trade-offs without compromising key elements. Plus these are moving targets... however FreeNAS drivers move slow, so our window is pretty large and not bleeding edge.