I think Grados are popular because of their mids, which are rich and flat and fun. Human beings are midrange detectors. The midrange is where we live. Most of the sound that we associate with our existence comes from the midrange. It's the sound of human speech, the sound of most instruments, the "detail" area of the music, as opposed to "presence" frequencies at the top and bottom of the audible spectrum.
Grados do mids really well. They don't filter the driver. Now, there are consequences to this as you move up the frequency spectrum. There are also consequences as you move down. A typical complaint of Grados is that they spike too much in the high-mids and treble while they provide too little bass response. What bass they do provide isn't the lowest bass but something around 100 Hz.
Even so, Grados are fun because they strike an interesting compromise between accuracy and excitement. Because they're open and because they're unfiltered, Grados do provide a detail and accuracy that you won't hear through more filtered headphones. On the other hand, Grados are not accurate to a fault. Their presentation is EQ'd. They emphasize certain frequency bands. As it is, these are terrific bands for rock and a few other genres.
Another reason for Grado's popularity is price - at least at the lower end. They're not the cheapest headphones out there, and some would argue they're not the best buy, at least not anymore. But Grado gained a reputation by introducing headfiers to open sound at an inexpensive price. Grado is also one of the few companies that recognizes the sonic benefits of utilizing wood or aluminum in the shells.
Somewhere along similar lines, Grados are so accessible that modding one is a bit like modding a Volkswagen. Everybody has an idea of how much better the Grados would be if Grado made one change for the better. People personalize the portable and expendable joy machines of the lower Prestige lineup. As they do, there's a feeling of empowerment. In that sense, Grado may be as much a "lifestyle product" as Apple, Bose and BMW.