Sometimes I have found, for whatever reason, I need to finish a review later, or want to double-check something.

It would be nice to be able to save a review in a draft folder before actually submitting it.

(I'm still having problems posting reviews but)

I support this idea.

It takes a few days for a review to get approved.  So if you are going to finish it up within a few hrs, I don't see any reason why you just don't submit it then edit it when you have the correct info.