Nominally, anything that actually says what the reader/reviewer thought, enjoyed, connected with, etc etc. If they liked it, telling me what they liked is better than "Hey this was great, update with more soon!" or some such, as I've received a few times. A review is a review, but some really are better than others in that respect.
Something like what I received from Aya/GossamerSilverglow a few days back really demonstrates the better review type - detailed, constructive, asking questions about things and posing suggestions for errors.