I found myself flicking through the national geographic the other day and noticed how I was paying far more attention to the photos than I would have done a few years ago. The main thing I took away from the magazine is that a lot of these photos were good primarily because they captured something, or rather conveyed one thing strongly. The caption on the photos was almost a tautology, or purely to assign a name to something which the reader might not otherwise know the name of.
While many of the photos were good technically, the ones which didn't stand up to strict technical criticism lost nothing for these flaws. The primary 'sense' of the photo in conveying very strongly one these was so much more important in many of these photos that the majority of the photo (providing it didn't actually detract from the photo) was largely inconsequential. That isn't to say there wasn't technically great photos in which the whole contributed to a very clear depiction, but rather the technical contribution tended to be less important.

I think that perhaps the best contributor to this sort of effect is simply something that the reader would not get to see normally. This ties in with what attracts interest in a photo with the natural instinct to find what is new interesting. Given that the remit of the magazine to "increase and diffuse geographic knowledge" almost directly covers this idea of showing people new things it is hardly surprising. With the isolating boundary of geography the subject matter is 'new' as a matter of course.