I have been on Sarlona since launch. I don't play frequently enough to have acheived any capped characters yet. I have three mid-levels. I have one in a smallish guild, one in a larger. I enjoy being in both.
However: On Sarlona I have never needed a guild. I find decent PUGs whenever I am looking for one. I usually find a group that's friendly, helpful, and capable. Sometimes the group is made up of guilded players, sometimes not. But Sarlona has historically been very friendly, and that has been my experience. I hope that with the increased population this trend will only continue.
I think the likelihood of positive experience is responsible for Sarlonans choice to remain unguilded, maybe as much or more than individuals' negative guild experiences, cause IME those are usually pretty good too, once you find a good fit.