Microsoft adCenter just lauched advanced negative match, a feature AdWords has had for a long time now, to help advertisers better prequalify traffic. Now you can set Phrase and Exact Match negative keywords in your adCenter campaigns for even greater precision in filtering unwanted search traffic. Previously, adCenter only allowed Negative Broad. For more information about negative keywords as well as how to use them, please see this post on negative keyword best practices.
Another new feature you may want to check out is their insights into keyword conflicts. Use the new Negative Keyword Conflicts report to learn if you have set any negative keywords that are same as your actual keywords. The report can help resolve any conflicts and help improve performance. It can also helo explain why some of your perfectly relevant keywords have not been getting any impressions.