In the late 70s, when subway graffiti was at its peak, the 2 & 5 line was widely considered the most prestigious canvas in the system, in part because it showcased writers from some of the toughest neighborhoods in the Bronx and Brooklyn. By the early 90s the subways were essentially clean, and since then most traditional tagging with spray paint has been reduced to commissioned works on warehouses and delivery trucks.
I don't deny that the majority of graffiti was and is tasteless scribbling, and period shots of subway car interiors completely covered in felt tip markings make me gag. But I do get the impression that watching elevated trains was a much more exciting endeavor thirty years ago.
There's some amazing work documented in Martha Cooper and Henry Chalfant's classic 1984 book Subway Art. To get an idea of what you missed, check out a digitized copy here.