I think there are basically two kinds of trip report:
1) The informational. Rather detailed but mostly dry, objective and to the point. Useful but usually not particularly entertaining. I'd put my last TR (the one on Mihin Lanka) in this category and say that, even if people have complimented me on it and I will agree it was a decent TR, nobody really had any fun reading it.
2) The fun/tale/novel-like. Subjective, personal and possibly lacking in detail (or overemphasising some details and totally ignoring others). Perhaps not very useful when you want to choose a carrier over another or know whether your favourite drink is served in the lounge, but interesting and often entertaining reading. Goldflyer's epic report is the archetypal example of this category.
Obviously, to make matters more complicated, there are a lot of reports that are a fusion of the two kinds. If you wanted to find an "objective" measure for what constitutes a "great report", you would have to combine the two...
But, at the end of the day, it's in the eye of the beholder. Depending on YOUR mood, a great TR may be the one that saves you from the grilled cockroaches in the business class meal or the one that makes you laugh for hours, or one that does both...