I think that a good trip report is one that provides you enough information so that you can safely duplicate the trip taken by the person who created the trip report.
The trip report would provide:
1. Trail head locations for entering and exiting- Date of Trip/Season of Year
2. Elevations of the trail head, passes, rivers…major land marks
3. Temperatures/weather encountered when the trip was taken day & night
4. Location of important water sources along the trail, pointing out dry spots
5. List guide books or maps that would be needed/useful for taking the trip
6. Photos to show the terrain and conditions to be encountered and to inspire
7. Photos showing gear used & taken to complete trip safely
8. Commentary to give a sense of what it was like to be on the trail, trip
9. Commentary aimed at the reader as tips and suggestions to make the trip better
10. Honesty in commentary, showing the good and bad moments
Maybe good timing, but here is an article that I wrote that was just published on this topic. It talks about how I take photos on the trail, what I am looking to capture in those shots, and the process of editing and posting online.
Not that I think I know everything there is to know on this topic, but it might give you some insight to what I think I am doing.
http://hikeitlikeit.com/
P.S. +1 On Eugene's trip reports. Love em!
-Tony