Good evening, everyone! Thanks for starting a thread for the 3D model support! Hopefully this will help to keep track of the most important issues. To answer the first question regarding the polygon limit, there is none other than the hardware limits. On older cards (read: very old cards, from 8 years ago or so) you only had 16 bit index buffers for geometry, which naturally limits all geometry to about 65k vertices each. On newer video cards, this limit is gone, making it theoretically possible to use multi-million polygon characters in Visionaire, haven't tried it myself though. I'd like to note that polygon limits in games mainly exist for two reasons: loading time and rasterizing time. Since the characters are the only rasterized objects drawn in the game, you will hardly ever use the GPU to capacity. Half-way decent video cards of the last ~3 years have no trouble processing several million polygons at 60 frames per second. Thus, the only thing you should keep in mind is the loading time, which also involves some geometry preprocessing.
As for the second issue: I once encountered the problem with character models being rotated by 90 degrees to the front. I think it was when I exported a model from 3ds Max. The only significant difference between the Assimp Viewer and the implementation in VS is that the Assimp Viewer is based on Direct3D, which uses a left-handed coordinate system, as opposed to OpenGL, which uses a right-handed coordinate system. If there's a problem with model rotations, it is most likely an issue with the definition of the coordinate system (although this should actually only invert the y-axis). I don't know if you can specify the coordinate axes in the exporter in Blender - can you? There is already a global scale value for 3D models in the editor, maybe we could also include 3 optional rotation angles if this is a problem that cannot be solved (conveniently) in the 3D programs.