You kinda have to read some of his work to know, and it does feel like a secret club with fairly exclusive membership - I don't really belong, I've only ventured into the shallows thus far :)
But the deal is:
Prose that creates a jungle-like density of meaning through the use of long sentences, creatively unusual descriptions using unexpected combinations of words with meanings just slightly twisted to fit into the size of the 'box that's available'.
Prose that acutely both describes and evokes the feel of specific times and places.
I probably miss more than half of the subtlety of his work through lack of familiarity with the 'time and place' stuff. I enjoy the structure created by the delicately woven words, however.
An abridged quoting of Hunter S. Thompson for a particularly memorable evocation of time and place (because this quote is more coherent and memorable than Pynchon):
Strange memories on this nervous night in Las Vegas. Five years later? Six? It seems like a lifetime...San Francisco in the middle sixties was a very special time and place to be a part of...
History is hard to know, because of all the hired bullshit, but even without being sure of "history" it seems entirely reasonable to think that every now and then the energy of a whole generation comes to a head in a long fine flash, for reasons nobody really understands at the time--and which never explains in retrospect, what actually happened...
There was madness in any direction, at any hour. If not across the bay, then up the Golden Gate or down 101 to Los Altos or La Honda...You could strike sparks anywhere. There was a fantastic universal sense that whatever we were doing was right, that we were winning...
So now, less than five years later, you can go up a steep hill in Las Vegas and look West, and with the right kind of eyes you can almost SEE the high water mark--that place where the wave finally broke and rolled back.
