Yahoo Answers is a great service, if you're comfortable with it by all means go ahead and use it.
Yedda is designed to make knowledge exchange easy, efficient and (gasp!) fun.
We believe that if you have a question, you should be able to ask it. So go ahead and ask as many questions as you need to - you won't find yourself limited by credits or points.
We also feel that when you have a question, you should be able to ask it even if you're not sure which category out of 200 categories it fits under. So we made it real easy to tell us what are the topics of your question, using free form tagging (i.e. topics).
But even tagging, while an improvement over "directory trees" (e.g. Taxonomies) is not always that easy. Which is why we invest a lot of effort in understanding what your question is about, and providing you with suggested commonly used topics. Hopefully this makes the process of asking a question as easy as possible.
And when you do send your question to Yedda, we know that as far as you're concerned, it's probably the single most important question in the world (well. Aside from "How to achieve world wide peace" of course). We don't think that questions should go into a "popularity contest", losing their place as newer questions are posted.
So, we created the Yedda Active Broker (temporary name!), which gets attached to your question the moment it is posted - a component which is rather single-mindedly obsessive about getting an answer to your question. This is the part of Yedda which is responsible for relentlessly searching through our lists of people for the right person to give you the best answer.
Plus, there is of course the ability to easily embed formatting, links and graphics in your question when needed. Hardly a must, but we just hated not being able to do that.
As for answering questions, with so many questions out there, and on so many different topics, finding that question which you'd like to answer because you think you can help out someone out there can be a rather chaotic process. For example, if you're a tax expert, and you happen to enter the system just a few days before the next Harry Potter book is out, you probably don't want to wade your way through hundreds of Harry Potter related questions. And vice versa of course 
So, once you tell us what your topics of interest and knowledge are (using My Settings > My Topics), we will find these questions which we think you will be able to help with and send them directly to you (of course, you can define how many and how often, if at all). We perform the search in an associative (or "semantic") manner – for example, if you specify "Home Improvement" as your interest, we understand that you might be interested in questions related to do it yourself, power tools, etc. (and if we get it wrong, tell us, and Yedda will automatically adjust itself and will better target your interests).
There is a lot more in Yedda, and a lot more coming up. But the bottom line – we're single-mindedly serious about exchanging personal knowledge, and we are trying to make Yedda the best place for this.
And, if you comments or ideas about how we can do better, please tell us (here, or through the Feedback link, or at feedback@yedda.com).