There is no such thing as the "average photographer". We need to know how people will be using a camera to understand what camera may or may not be too much for them. I don't think we know enough about "Regulator75" from this thread to know if the D300s is too much camera. It may be. It may not be.
I think the D300s is ideal for the serious amateur. One thing to point out is that the D300s has many features the serious amateur doesn't know he could use, would use, and needs. The serious amateur would grow into the camera learning how to use these features and, in the process, expand his photographic toolbox.
The sensor is just one part of the package. Yes, the D90 can take pictures that look just as good as the D300s. But the D300s makes that a more productive process if you understand how to use it.
Maybe I'm wrong here, but from what he showed us, he seems to primarily shoot still subjects (landscape and macro). The (valid) points you brought up in favor of the d300s I don't believe would nesc. help him;
"The D300s sensor is faster at moving data which is needed for the high frame rate"
-wouldn't help for landscape or macro, and as he said he doesn't shoot sports or wildlife
"Where the D300s shines is in AF performance, meter performance, and in usability."
-AF performance doesn't seem to be something he needs
-meter performance I think is something that may come in handy, but really I don't think justifies buying the 300s over the 90.
the only other possible thing you mentioned that seems like it would be a better feature for him would be the higher bracket amount (for hdr), but judging from his pictures he doesn't do that either.
also you called the scene modes ridiculous (and while most of us would probably agree) as he mentioned that would be something useful for when his wife wants to use the camera, so in this case it does affect things.
so while I think everything you said is correct, based on the information regulator did supply us with, I think the 300s might be a bit unesc. for him with the work he is currently doing.