Thank you for the feedback regarding commenting on merge requests. The diff view is indeed very important for collaboration. In the upcoming 9.2 release (slated for May 22nd), we are adding the ability to comment on older versions of the code and diff between versions.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10388

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10572