I write lots of Battle reports for my blog, and in the main they have a general introduction to the background to the game, a run through of the setup, terrain etc (this bit is often quite long) . Then a run though the battle itself, and finally a wrap up. I sometimes highlight specific units, particularly if it is their first outing or if they are very venerable.
It is largely guided by what photos I took and how much effort I want to put in. No idea if that is the best approach, but it works for me and my readers apparently like it.
"Mistakes in the initial deployment cannot be rectified" - Helmuth von Moltke
I find that it varies with the writing style of the author. One type of AAR written by one person will work for me, written by another in the same style, it will not. I generally like a story form, rather than an description of the rules mechanisms and die results, but again, it is all dependent on the author.
Some photos are always nice, hopefully both overalls, and some close-ups for a little atmosphere.