Below, AnneArchy makes the point that the Real World ushered in the era of reality television. I know that some people will point to all sorts of other formats with reality aspects that existed prior to the Real World (game shows, documentaries, etc.), but I think that modern reality--especially the slutty girl + obnoxious guy + alcohol = drama and ratings formulation can be traced directly back to it. Interstingly, I don't believe that the Real World has ever had the ratings comparable to its major network descendents, but its influence is pretty clear.
Of course, this leaves the question of whether this is a bad thing. One is tempted to say, "yes," as one tends to think of alternatives like Arrested Development rather than According to Jim--in other words, reality tv isn't necessarily replacing something that's all that better. I will posit that reality tv has been wonderful for trash television, as it's hard to replace the illicit pleasures of things like My Big Redneck Wedding and My Bare Lady with 3rd rate sitcoms and soap operas.