I've been playing with Quora a little bit. I think it's going to be big...here's why:
It fills a gap.
Twitter's 140 character limit is too limiting, especially for meaningful discussion concerning meaty topics. The answer isn't always immediately obvious, and something like Quora allows users to really express their thoughts and evolve them over the discussion.
Quora is also a good way to establish thought leadership and credibility in theTwitter universe. The Godin's and Brogan's of the world don't have this problem...but the rest of us (who are somewhere closer to obscurity than fame) do. The reason Quora can do that is because it's built on top of Twitter's social network. Quora adds an additional layer for professionals (where expertise is an important element of reputation) in much the same way that YouTube would be useful for an aspiring filmmaker.
Twitter is becoming a raw information stream.
Following trends on Twitter can feel like stepping into a firehouse, or trying to cross a freeway in rush hour. Some people are connecting and talking in short bursts while others are broadcasting different types of content. It's all quite hectic and intimidating to the uninitiated but when it gets formatted in a meaningful way we can use it to build connections and see relationships.
I envision more services building themselves on top of Twitter's architecture. For example, when I want to share something I'm reading with Flipboard, I just click a button and it automatically tweets it to my followers. As services build themselves on top of Twitter's architecture, we'll find that it becomes an extension of everything people are doing online and the backend for a lot of good social stuff.
I hope that makes sense. Let me know if anything needs clearing up in the comments. In the meantime, I'll be tweeting a limk to this and building a reputation on Quora. See you there!