The arXiv does not currently allow comments by its readers and authors. The idea is that the arXiv is not peer-reviewed - peer-review happens elsewhere, at the journal level. Thus, a comment and reviewing system is hard to maintain and run, and not useful. Yet, preprints offer an unprecedented opportunity to first, open up, and second, increase the quantity of reviews and comments that manuscripts go through. We do not advocate to replace traditional peer-review, but to complement it with open review of preprints. We believe that (1) more scholars should participate in the peer review of an article, and (2) peer review should be done in the open, so that the review itself becomes a crucial component of the published (or pre-printed) research. This seems to be not only natural, but also necessary given the ever increasing number of publications and average number of authors per paper, which renders the current peer-review paradigm unsustainable. The authors of this document welcome public comments and ideas from its readers, at the online version of this article. The arXiv of the future will allow open comments and reviews in addition to traditional peer review.