The main difference is that Fluther pretends to be something it’s not. The vision (I assume, at least) is to create a place where someone can ask a meaningful question that they can’t find an answer to online. Their question would then be answered by real people who are knowledgeable about the topic. Then, that question would appear in the results of search engines. It’s sort of like a human search engine. You ask a question, you get an answer, then that answer gets recycled to anyone who might ask the same question later on down the line.
This is why you see such an emphasis on the “quality” of questions, a general negative attitude towards socialization, and a strangely fervent pride in moderation.
However, this scenario is highly idealized. It’s not as if you have to pass some sort of proficiency test to become a member and start answering questions. You can label yourself as an “expert” in any topic you wish. There’s been a lot of talk about how Fluther “funnels” questions to you that are related to your fields of expertise. By this, they just mean that you get an alert when someone tags as related to a topic you’re interested in. A thousand other sites do this, it’s nothing different.
There is a points (lurve) system, which is intended to showcase how qualified or experienced each member is. However, there is very little to control whether people receive lurve for actually giving good answers or for just being silly. Also, lurve can only accumulate and can never be lost; thus, anyone who simply uses this site for a long enough time will acquire a large lurve total, regardless of whether they submit helpful answers.
There is definitely an innocent, social, forum-like component to the community, just like any other Q&A site. The difference is that Fluther tries to cover it up and appear to be more business-like, especially when they are snagging large investments and being featured in web articles. Now, the emphasis on moderation and question-quality does tend to weed out the 12 year olds and trolls a little more than most sites; that may be a good thing or a bad thing, depending on your outlook.
Although there are varying opinions on Fluther’s aesthetics, it can safely be said that the site is sufficiently fast. There are a handful of features, such as live commenting and answer previews, which make the site easy to use and navigate.
If you really want to see what makes Fluther different from other sites, take note of the inevitable responses to this comment.