We recently uploaded a preprint to bioRxiv. The goal was to hopefully get some constructive feedback to improve the manuscript. So far, it got some tweets and even an email from a journal editor, but no comments or constructive feedback.
I notice that very few preprints on bioRxiv have any comments at all. Of course, scientists may be emailing each other privately about papers on bioRxiv, and that would be great. But I think a open process would be valuable. F1000Research, for example, has a totally open review process, posting the referee reports right with the article. I might be interested in trying that journal someday.