Is there a good general term for "the thing that is being reviewed"?
The activity is reviewing
The product is a review
The person conducting doing it is the reviewer
The object being reviewed is ?
The closest I've come up with is specimen, but that is more the subject of scientific scrutiny. I could use a more general term like subject but I would have to prefix it to make the context clear (e.g. review subject).
As I read the question, really the subject line, the noun subject came immediately to mind. If you need to clarify, and I am not sure you would, the subject of our review, or words to that effect, ought to do it.
The activity is interviewing
The product is an interview
The person conducting doing it is the interviewer
The object being interviewed is the interviewee
Using the stem of the word, it is likely that the appropriate word would be reviewee, although it seems to refer specifically to people undergoing a review rather than absolutely any subject; if you're going for consistency between words, 'reviewee' seems to be the obvious choice (even if Chrome's spellcheck flags it as a non-word).