Essentially the best option for drawing Polygons would be to determine their "resolution" based on size. So make large forests appearing at lower resolutions than small forests (well I think we all know that best would be if resolution of any element were adapted to map density, but I think that is even more complicated). I don't think this would be an easy task. Depending on the area or country (e.g. France with the Corine land cover import) putting forests at low resolutions really slows down map panning. An "easy" compromise could be to put "stronger" douglas peucker filter on polygons than on roads (I currently use 5.4, because 10 seems to be too strong on roads). However for Polygons a value of 10 works pretty well. It would be great if there would be support for that (I don't know if the douglas peucker filter is applied after or before the style-file, if before of course the above will not be "easy"). The better mapped countries are getting, the more importance choosing the proper resolutions will have. The changes to the default style over the last weeks were already a good step forward, but outside of cities OSM rarely has more than 20-30% of ways and landuse covered (I dare people in future will not draw few big forests, but split up more and more detailed) so the solely based on tags approach will become more and more difficult... (well at least I hope the difference between countryside and cities will flatten out a bit).