There is some kind of standard for writing reviews from introduction (although nobody cares for it), a discussion (exhaustive list of details to make everybody happy) to the usual conclusion that further research is required. When writing now something for Bioessays, I found a completely different recommendation that I believe is a good approach.
An abstract must immediately communicate what is new, not give a general statement along the lines of “in this paper we review field X”. Reviewers will generally be experts in the area, and will be interested in new insights. Bear in mind the following:
Concepts placed at the beginning and end of the abstract will be more memorable than concepts placed in the middle. So, place your most interesting and new insight as high up the abstract as possible, and end it with an interesting forward-looking or speculative observation.
Material in the middle of the abstract will be less memorable. So, do not place novel and important things in the middle of the abstract; instead, use it for contextualising background.
The principles stem from the application of the ‘serial position effect’, noted by researchers who studied how well people remember concepts placed at the beginning , middle and end of a text, see Deese and Kaufman (1957) and Murdock, B.B., Jr. (1962).