Standard Q&A sites consist of one question and a single set of answers. They're too limited for long and complex discussions with many tangents. Yoomoot doesn't have this limit: it's a web of interconnected questions. So it allows for complex open-ended discussions whilst still retaining the organisation of the Q&A format.
Avoid asking question that are subjective, argumentative, or require extended discussion. Stack Exchange does not work well with questions like "Which is the best..." - source: http://area51.stackexchange.com/faqYou can see what a long conversation looks like Yoomoot-style by looking at this discussion about the 'Zeitgeist Movement' via the discussion browser (click 'open discussion browser'). It's not possible to have this kind of discussion in a Q&A format anywhere else.
Other Q&A sites are have a "get your answer and go" focus. In contrast, Yoomoot is focused on ongoing conversations. As such, we've made it easy to keep track of specific changes, but in a way that doesn't overwhelm you.
For details, see The collaborative inbox: How Yoomoot makes it easy to keep track of updates and discover relevant new content
If you don't know the name of what you're looking for, but have an idea about what topics it relates to, you can use the categories browser. Instead of a traditional hierarchical categorization system, Yoomoot recognizes that the human brain works by association. The same thing can be associated with multiple categories, and a single category can be associated with multiple other categories, in a non-hierarchical way.
Other Q&A sites are organized either by a traditional imposed hierarchy or by a mass of disorganized tags without any way of browsing the connections between them.
Yahoo! Question has the feel of one big forum about everything.
Stack Exchange completely separates different topics into different sites.
yoomoot takes the middle path. Many, perhaps most, questions fit into multiple subject-areas. Forcing different subject-areas into different sites means having more-or-less the same discussion in multiple places. This kind of needless repetition wastes people energies. It also creates unnecessary effort for people interested in multiple subjects. Isn't it better to have a single interface for accessing all the discussions you're interested in?
On the other hand nobody is interested in everything. Yoomoot is organized around 'favorite categories'. Once you've decided what specific categories are of interest to you, you can pretty much ignore the rest of the site. The end result is a network of overlapping niche communities.