All art, at one point in its life is modern.
Michaelangelo, Da Vinci, Rubens, Constable, Cezanne, Monet, Picasso, Rossetti etc., all faced criticism of their work from the established art world at the time. Granted we all now know them as works of genius - but there is a fine line between genius and madness.
If you look at some works by Constable, they're amazingly modern. One, entitled "Seascape study with cloud" would make you stop and say "wtf" - it looks like a child has scrubbed his paintbrush across the canvas - yet it is worth a small fortune. It isn't worth that in paint or canvas or age really since there are older artworks out there, its worth it due to the person who created it.
Photography is a 'relatively' new medium to work in for anyone interested in Art - it sometimes has a lot of problems being accepted as "art" since in many artists eyes it is just a case of pressing a button to make a picture.
It is more than that though - composition, using the available light, timing, subject are all captured by the camera. If you have used film, the type can make a difference - Black and White, Slide film, Colour Film or even Polaroid. Then you have the developing of the film or alteration of the shot if Digital and finally the print stage, even the type of paper you print on makes a difference. Each and every one of these things results in a unique image.
Now it might take you 10 minutes to output a shot, or it might take you a few hours as you work on it then print it. So what is it worth? To a parent a photograph of their new born baby is priceless. If it isn't my baby its just another shot of a baby, it might make me go "awww" but thats about it. Worthless to me. Worth is open to interpretation.
Now the Gursky photograph. He has taken the time to come up with something unusual. Sure some people probably took similar pictures, but he has made it his own style. If anyone else took a shot like that, it would be referred to as being in the 'Gursky' style or 'Gurskyesque". At the point that epithet came into being, Gurksy's work became valuable.
The same goes for moody black and white photographys of American Landscapes - Ansel Adams, or how about Candid Street shots - Henri Carter-Bresson? We now compare candid shots and landscapes with these photographers, but people were and in fact sometimes still are, very critical of their work. Again it is all down to taste. Do you like it? What is it? Does it work?
There are billions of pictures out there - including the ones on these forums - yep they're technically very good - sharp, well exposed, punchy colour, great action shots, lovely wedding photos, amazing macros - but they aren't iconic - they're the same as all the other shots out of a similar nature and that is why they'll never command six figure sums.
If photographers call photographic work crap then what hope is there for the medium to be accepted as 'art' by the mainstream?
Oh, one last thing - take a look at this vast list of Photographers - might broaden the mind a little
http://en.wikipedia.org/wiki/List_of_photographers