Does a preprint include revisions made in response to peer review?
I am using Elsevier as example since they are often picked on for many of their policies. Elsevier attempts to be clear about their sharing policy and allows both preprints and accepted manuscripts to be shared. In "Elsevier speak" (which may not agree with standard usage of the terms in academia)

Preprint


  This is the author's own write-up of research results and analysis that has not been peer reviewed, nor had any other value added to it by a publisher (such as formatting, copy-editing, technical enhancements, and the like).


Accepted manuscript


  An accepted manuscript is the manuscript of an article that has been accepted for publication and which typically includes author-incorporated changes suggested during submission, peer review, and editor-author communications. They do not include other publisher value-added contributions such as copy-editing, formatting, technical enhancements and (if relevant) pagination.


Elsevier allows for "accepted manuscripts" to be made available via "preprint" repositories (e.g., ArXiv) which of course speaks to how confusing the terminology is. So at least for articles published by Elsevier the difference between the published version and the version in a preprint repository might be limited to "copy-editing, formatting, technical enhancements".

That said, and one of my pet peeves about preprint repositories, is that the "preprint" may have any number of non-peer-review changes. Maybe during the review process some controversial data or conclusions were removed from the manuscript. There is nothing preventing the authors from re-inserting this into the "preprint".