build quality and f-stops isn't close to all that is different. Actual light transfer (e.g. t-stops in cine, but less quantitatively in SLRs, say - some lenses

build quality and f-stops isn't close to all that is different. Actual light transfer (e.g. t-stops in cine, but less quantitatively in SLRs, say - some lenses are brighter than others), sharpness, chromatic aberration, spatial aberration, focal speed and accuracy (assuming autofocus), consistency across the telephoto range (not for primes, obviously) - some lenses will have not just differet f-stops at different lengths, but really different characteristics. This doesn't even get into the subjective aspects (e.g. "bokeh") that some people care a lot about.