(My guess is you wanted to write "Can't I post any Open Questions I have right now...", so I will respond to that, but let me know in case I misunderstood)

Yep, you can, but I think there are a few reasons why people don't, and why doing so wouldn't get you all the benefit of having a more dedicated Q&A system:

• In terms of UI, I think you want to give people affordances for asking questions, which we right now don't do (simple things like having a smaller text-field that doesn't scream for 3 pages of content, plus a top-level button that says "Ask a Question"). Having dedicated UI for this, with centrally placed buttons, I expect will significantly increase the degree to which people think of interesting questions to ask.
• I think question and answer pairs have the opportunity to be better indexed and longer-lived references for important ideas than blog posts are. Similarly to how StackExchange questions often have discussions over multiple years, I think a Q&A format on LW more naturally lends itself to creating more timeless content. But to take advantage of that, you need to have some UI that makes that historical content accessible, and that encourages users to point to previous times a questions was asked, instead of opening a new thread.
• One of the ideas that has been central to a lot of our thinking on this is the question of how we can measure and incentivize long-term intellectual progress on important issues. In mathematics and many other sciences, well-known "Open Problems" (such as the Millenium Problems, or the problem of squaring the circle, or the problem of dark matter) have often served well at organizing research fields and helped ensure that fields continue to make real intellectual progress. Similarly, the goal with this system is to allow users to create common knowledge about important open questions, and to help people organize all the progress and research that has been done on those questions so far, and I think the current post-system isn't amazing at doing that.
• I am interested in experimenting with incentives for answering questions, in the form of Karma, but also in the form of prices and financial rewards (similar to Paul's "Could we see distant aliens" question + prize). I think to do that you need to be able to distinguish discussion from the answer, have the ability to mark things as closed, and some infrastructure for people to attach various bounties to questions in a way that doesn't really make sense for posts. (In a bunch of user-interviews we've found that many people would be interested in doing similar things to what Paul did in that thread, but were unsure about whether this would be welcome on LW, or didn't want to deal with all the logistics associated with awarding prices, both of which I think we can address)
• In terms of voting-patterns, I think right now people tend to upvote things if they think the content of a post is good, and told them something new, which is relatively rare for open questions, so I think they don't tend to get as much attention as they deserve. I think it helps to create a separate mental category for users for which they can ask reliably themselves "is this a question to which I would like to know the answer to?" as the primary question to ask before upvoting.
• I think right now if you ask an Open Question, and your question does get a bunch of attention, the answer will come in the form of a giant comment-thread that will probably be 5 times as long as it has to be, which is probably good enough for you as the person asking the question, but is much harder to link to and reference in the future. One of the goals with the new format is to encourage members to distill the content of a discussion into a new top-level answer that includes all the relevant info that you need to know, without needing to dive into the depth of a comment thread. A lot of the goal with the Open Questions system is to distill content into more modular forms, with the goal that this will make referencing past discussion and building on past intellectual progress easier.