I think that a lot of it is is a case of undocumented behavior which doesn't make sense to document properly, because a lot of stuff is placeholders and subject to change and that will render any documentation obsolete pretty fast
Most of the times, problems are either due to directx version used, .net libraries and people not realizing that running at full graphics is impossible if your PC doesn't exceed the recommended specs.
It runs fine on my two year old PC:
i7 920 @2.7 Ghz (stock clock speeds)
Ati 4890 1GB
3GB RAM
Win7 x64
I use mostly medium settings, a couple of low settings and a couple of high ones, running it at 1680x1050. Runs fine and stutter free at 25-60 FPS (capped due to Vsync) depending on the terrain bellow me. The only way to make it start having hiccups is fly very low (like rooftop level) over London, otherwise it's all fine.