As a developer, I frequently go through pull requests and leave questions for the author. While I await these answers, there is no real way to indicate that I've otherwise completed the review. Marking something as "Approved" or "Needs Work" seems incorrect if all I really want is answers to questions of mine before I give final approval. 

I suggest adding a third option to the pull request status, called "Awaiting Response" (or maybe "Needs Information" or something similar) to indicate that the reviewer just wants the pull request author to answer some questions and not that there's obvious defects or problems in the code.