1) To call the premium models (and I assume that's what you meant with "20%") "acceptable", is completely untrue. In fact the premium models have ridiculous detail that you hardly find in any other game. I would say 20% of the cars were fantastic, and on the other hand
some of the standards looked quite bad. I can't give any reliable number on that though. I actually would say that many times I saw a car on track and did not immediately believe it was a standard. And that is the definition of "acceptable" for me.
2) For a long time it was unsure, if there even would be time and weather change in GT5. When it finally was in, and offered spectacular views, of course now people come and say, why not all tracks? Well, same as for cars ... because there's never enough time for everything. Plus, it doesn't make sense on some tracks. E.g. the SSR5/SSR7 tracks are permanent night tracks, so it doesn't make much sense to add time change, because you would have to create a whole scenery first.
3) Again, it needs extra work to create racing modifications for cars. Plus, you might not have been able to create RM's out of many cars, but for many types there is a racing model in GT5. Doesn't make much sense to add an RM for these then is there? E.g. there is a 350Z/Fairlady race car. Don't need racing mods for 10 different 350Z's then, do we? Ford GT Test Car is like an RM, there were race models of all Skylines, etc. So, your count is flawed.
4-6) Your other points I can only agree with.