A lot of territory has been covered in this thread and all the observations are on-target (except those rationalizations for not detailing ~ that was not the topic. No one has suggested anyone MUST detail a model.)
A couple of the most frequent but most glaring flaws have already been mentioned ~ [1] visible injector pin marks (especially on interior floors and chassis), [2] decals spanning door and panel breaks, [3] visible mold parting lines on oil pans, transmissions, radiators, small parts, [4] wildly mismatched panels, doors, hoods {see Tom's '57 Ford in the post above - sorry, Tom}, [5] grotesquely oversized spark plug wiring {especially when it looks like garden hose connecting distributors to spark plugs}, [6] absence of chrome on wipers, door handles, lock cylinders, emblems, and window trim, and [7] absence of blackwash on grilles and cowl vents, etc.
Add: [8] visible, unpainted areas (such as undersides of hoods or interior buckets that show in transmission tunnels, [9] unfilled or untreated gaps in seats, and [10] sloppy gluing, smudging, fingerprinting.
From my experience, those are probably the Top Ten factors (along with careful preparation, clean building, nice paint, and pre-build research) that make and demonstrate the difference between mediocre builds and good, very good, and great builds.