Modulated is like translucent, but where a surface gets more opaque where it is brightly lit, and more translucent were it is dark, the oposite happens with modulated, ie. dark spots are opaque, and lighter spots are more transparent. If you look at a modulated surface in textured mode it will appear invisible because it is at full brightness and therefore wholly transparent.
Select Modulated+Mirror to get a nice effect
Force View Zone seems to make the surface unlit, I don't know if it does anything else aswell.
2 sided does just that, gives the surface 2 sides. Try subtracting a cube, then moving the camera outside of the cube, you won't be able to see the surfaces that are facing away from you, select one of these surfaces and select 2 sided, you will now be able to see that surface from either side. The surface keeps the same properties on both sides, same texture, same alignment, same light map, hence selecting 2 sided will not increase the overall polycount of the level, but it will mean that the surface may be visible from places that it was not previously, so it may increase the poly count in view in some places.
Also, a 2 sided surface uses the same lightmap for both sides therefore if it is lit from only 1 side, and not the other, the side that is not facing any lights will still have the same brightness as the other side. You should be careful of this because if the surface is a window for example, it will be getting lit from both sides and it is possible for it to become overly bright.
Errr, high shadow detail and low shadow detail speak for themselves, except you should note that checking both of them at the same time will result in SUPER low shadow detail.
High shadow detail will result in a lower framerate, especially if there are any dynamic light effects Eg. the light given off by a shock secondry fire or fire waver light effect etc...
No smooth stops the surface's texture getting smoothed when you get close to it, you can see the effect in the editor.
Special poly doesn't seem to have any effect...
Dirty shadows doesn't seem to have any effect either
No Bounds Reject, guess what, can't see any effect
Errr yeah, that's most of the more obscure ones I think.