I like the StackOverflow interface so much I am now disappointed when I go to a blog and cannot vote up or down groups of comments. It bugs me even more when I go to http://blog.stackoverflow.com or Coding Horror and the site just seem off. I am hoping that Jeff and Joel hire someone like they did for Stack Exchange and adapt the engine for blogging.
Voting is clearly important, but I'd also love to be able to use markdown, particularly for coding blogs.
When I blog, it typically takes a little while to phrase it (especially if I need to add attachments). I suspect you'd need pending posts (visible to the author only) that you can edit incrementally before finally publishing it.
If you wanted to get fancy, delayed publish so you can blog while on holiday ;-p
My number one request would be for having groups of comments be like an answer and be able to vote up and down the comment.
Ideally, this would be a WordPress plugin which supported both threaded comments and comment voting. In a perfect world, it'd even support point-based reputation that gave commenters increased moderation abilities.
Anyone could just drop the plugin into their WordPress, and automatically get SO-style comment voting/threading/moderation.
Markdown could be offered as a separate WordPress plugin... in fact, I think one's already available: http://michelf.com/projects/php-markdown/