there is so much mediocre photography out there
but in the end photography, like all other art, is completely subjective. it's just whatever you like.
i mean, i can look at a photo and think "that photo has good/bad composition" "that photo has good/bad contrast" "that photo has interesting/boring subject matter" "that photo is in/out of focus but in the end i guess it's just a holistic personal preference
some photographers become famous because of their distinct style (man ray, juergen teller) or the industry/context within which they work (ellen von unwerth, anne liebovitz), or their subject matter (dorothea lange, ansel adams)
i dunno i'm just rambling now
basically comes down to "how does this photo make you feel"