Clouds seem a bit too dark colored to me, but I'm impressed with how natural the vegetation looks. Colors of imported objects often seem overly bright and saturated, but can look too "dead" and dull if saturation is reduced. This seems like a very good, realistic balance.
I too would be interested in seeing the source files for a scene where populations won't work. So far I don't think you've described even the specifics of the issue beyond saying they simply don't work, so we don't know whether you have any error messages, or whether the objects show up but are floating, or some other issue. More details would definitely help us resolve the problem.
- Oshyan