I have to assume that guys who detail for a living all the time must take a few shortcuts (not obvious or even noticed by the customer) to save time. The net is you get what you pay for so in terms to attention to minor details.
For example, I saw some videos on autopia by one of the regular posters (someone who does it for a living) and I was surprised how little he cleaned the wheels (between the spokes) with a brush. My guess he works on well maintained cars more often then not.
For example, a full detail for $150 will not be as through as a full detail for $300 even though on paper they are essentially performing the same basic tasks.
I think you are quite right about what is done and what needs to be done being a pretty subjective thing.
A lot of the things that some of us think are necessary aren't even noticed by many/most people.
Things noticed: Not necessarily in any particular order.
Shiny paint
Clean windows
Clean face of wheels
Vacuumed carpet where easily seen
Clean dash and instrument panel.
Things not noticed by a lot of people
Minor marring usually called spider webbing or swirls in the shiny paint
Top edges of windows not cleaned
Inside of wheel and spokes not cleaned
Under seats not vacuumed or blown out with air hose
Nooks and crannies of dash and interior not cleaned
Trunk not vacuumed
Underside of deck lid not clean
Door jambs and sills, trunk lip, underhood painted areas not cleaned.
Most don't even know that an engine can be clean
Dressed tires and wheel wells are not noticed by many
Some comments I have received over the years that lead me to believe that the little things we do for ourselves are not all that appreciated by the general public. Even those involved with automobiles in some way.
Parts man checking to see what hose I wanted to replace: "Do you ever drive this thing?" Vehicle had 100,000+ miles on it at the time.
Employee at homeless helping unload colthing from our van: "Did you just get a new van?" It was 4 years old with over 50,000 miles on it.
Employee at dealership doing a warranty repair: "I'v never worked on an engine that was this clean."
Friend that I had told I was going home to clean up the vehicle I was driving as he walked around the car: "Just what is it that you think needs to be cleaned up?" To me, the entire vehicle needed to be washed and the interior cleaned.
Another area that has surprised me is that I have never received any comment about my wheels being relatively clean on the inside when the tire shop installs new tires. I don't think they even notice.
My point being that the average customer will be happy with a lot less than most of us realize.
Eight hours may make a vehicle look better than new, but 3 hours can make it look good enough for most people to be very satisfied.
Actually, most of the ones I work on these days are either my own or family, so I can spend a whole lot of time on them and not get paid for it.
