Lots of good observations, many that I can relate to.
Lets forget about the engine, chassis & interior detail for a moment & concentrate on the exterior.
Stance is everything. Whether it's a gasser or a lowrider & all points in between, how it sits on the table is crucial. Many people mentioned wheels being lined up, centered in the wheel wells, & all of that plays into it. But, how about sanding the bottoms of the tires flat? Practically every model that I see (including 99% of those that I've built) has tires inflated to gravity defying proportions. Although sanding the bottoms of the tires will not produce a realistic sidewall bulge, I think it's still better than perfectly round tires barely contacting the ground surface. This is one of the things that I learned like Ace-Garageguy did, using a camera.
Valve stems can be a be a very nice addition, but why bother if the thick rim of the wheel sticks out of the tire a scale inch or so? I'd spend time getting the wheel to sit realistically in the tire instead of adding valve stems.
Panel lines really don't need to be darkened much as long as they are scribed deeper.
What's the point of having opening panels if it looks like a wreck with the panels closed? Rounded edges on doors, hoods & trunks that don't close properly & have a 1 or 2 scale inch gap. I prefer seeing a well done properly scribed curbside.
The thing I see most often is body colored vent window frames. I can't remember ever seeing a 1:1 with that feature, but I may be mistaken. Well, some trucks have black vent window frames, maybe white, too? Regardless, I know that applying BMF to these can be a pain, but unfortunately, I think it's necessary. I've also seen models with body colored vent window frames, yet they have chrome drip rails??
The same applies to body colored trim pieces (side spears, hood spears, etc.) scripts, badges & side marker lights. Sorta' like having a nice classic car repainted & the painter forgets to mask off a few things.