What makes Fluther different from the other Q&A sites that I’ve seen (and which don’t interest me enough to spend any time with) is that the questions become a continuing conversation between the asker and the answerers, with each person adding something to the whole. Presumably, this is because the answers remain in sequence; none rise to the top, breaking the flow.
And yes, the fun and clever things, like “crafting a response”, which @ucme referenced; like the Fluther greetings, and the awards, and Neptune eating deleted accounts, and Dr. J himself. There’s whimsy here; it doesn’t feel like ad-man-designed gimmickery.
And, although many of the first Fluther users have gone now, looking back at early threads, it seems to me that the spirit of the place came into being because the core users already knew each other a little – they were friends of friends or relatives, and just nice people; their characters shaped the community that became Fluther. Of course, it has shifted over time, but I think that the essence, the attitude of Fluther is unique because of those origins and the specific people who were here.