This has been said before, but I think it's worth reiterating:

 Yedda is  about mining people's personal experience. From my perspective, it's not about just knowledge, but about people's interpretation of the world - information vs. data.

In google you'll only find already-posted stuff and you'll need to look through it to find what you want. It's good for locating a resource on the net.

In wikipedia you can find a compilation of people's knowledge about topics, though not really a personal touch.

 In yedda you can ask for opinions, you can ask about localized stuff (where's the best store in XXX for YYY), and for knowledge that might be on the web but you'll have to sift through gobs of information before finding it.

Plus you'll get the dissenting opinion (minority report) if there is one, which none of the other services offers readily.

There's another aspect to this: Yahoo came out in the 90s and organized what was there on the web so that people will be able to find it. Google did the same for the current web. Yedda will do the same to personal questions. If it's been asked before, you'll be able to find the answers based on your question. Think about it as a learning experience portal.