I don't know. I actually quite liked "Elfstones." I felt like Terry Brooks managed to make his world much more unique in that one and gave it a sense of place instead of just Middle-Earth with the serial numbers filed off.
And you can't tell me that "Druid of Shannara" and "Elf Queen of Shannara" aren't both just awesome by themselves. That series as a whole is middling-to-fair, as the first and fourth books are mediocre at best and the overplot isn't really all there, but that stone city thing... man. That stuff is rad.
I thought both "Elfstones" and "Wishsong" were of low quality, trying to suck more money out of a concept that he'd taken as far as he could. I guess I was thinking of the original "trilogy" only when I referred to the followup books disparagingly. I haven't read Druid or Elf Queen since I was a teen but I remember enjoying those very much, because the writing and ideas just seemed much better and not just rehashes of the original, the Shadowen being a cool component especially, and the stone city. I particularly liked Walker Boh (I think that was his name).
I believe Druid was the 2nd Shannara book I ever picked up, having no money at the time and instead only able to grab whatever book was available at the library.