To those worried about polycount, remember that there are maaany ways to work around it.
Firstly, you use LOD models - the LOD stands for Level Of Detail. Simply put, you take a model of a barrel with high detail (6k polys) and create successively lower detail versions, up to a certain point (say, 100 polys). The engine then chooses which model to use, based on camera distance. So if you are zoomed up close an personal to a food stockpile, the high-detail versions will be used for graphicy goodness, and in an outside scenery shot the very low count barrels will be used.
Secondly, modern GPU's provide the ability to do instancing - that is where you load a set of meshes and textures (i.e. a model) into the graphics card's memory, and then you load up a list of positions, scales and orientations. The GPU will then use the same model, but just render it X number of times using hardware. This cuts the time spent transferring data between main memory and graphics card, which is often the speed limiting factor.
Lastly, there is a technique called imposters - where you render the model to an offscreen surface to get a 2D sprite (a transparent thumbnail picture), which you then 'stamp' wherever the model appears far in the distance. So the cost of rendering the model then becomes drawing a single quad (two squares). That is the technique VF uses for the tree models - you can see that after a certain distance the trees' appearance change somewhat, looking more 'flat' than usual. The Total War series of games relied heavily on the imposter system as well - otherwise it would be impossible to render 5k+ troops. It explains the spritey look of the soldiers in the early games from that series.
AFAIK OSG supports all three techniques mentioned above, so I am really not worried about performance at this point.
Remember that a big part of Obsidian is its planned ability to export to common raytracing renderers (like Povray, Blender etc.). So we'll eventually need a high-detail raytrace and lower-detail real-time version of each module (and texture?).