Nils,

I think that an experienced observer will always see more through a scope than
an inexperienced observer--in all conditions.

So experience is important.

But I think sky darkness trumps experience, so that factor has to have more
weight.

But aperture trumps experience. A novice, looking through 60", will see more
than an experienced observer in an 8" scope.
[I mean actually see, not observe. I find that rank amateurs can actually SEE
more than they OBSERVE if you ask them pointed questions about what they're
looking at. The experienced observer more or less automatically observes what he
sees.]

So, ranked:
Aperture
Sky darkness
Experience

I consider pupil diameter, star color, distance from zenith, telescope type,
cleanliness and coating type, and seeing conditions all as lesser factors.

Upon reflection, aperture and sky darkness are close to equal in importance. My
home is about mag.17mpsas, while my observing site is magnitude 21.5mpsas. That
difference radically overcomes the difference between my 5" and 12.5" scopes.
At the dark site, the 5" sees more than the 12.5" at my home.
If the difference in sky darkness is larger than the difference in the
apertures, sky darkness is the more important factor.
But, since amateurs are unlikely to be able to experience 40:1 aperture
differences at their observing sites, sky darkness is really the most critical
factor for determinining what can be seen.

At a given site, though, aperture is the most important factor of all.