I was the one who made the first booklist, so I'll give my reasoning for excluding it:

The booklist's primary purpose is to be for interested laymen, and the works are supposed to be written at a level appropriate for roughly an undergraduate, but with some leeway on either side. They are supposed to be informative, as correct as possible, and generally provide a good introduction to the topic for someone who may only be familiar with the outlines (although quite a few assume considerable prior knowledge).

Recommending Gibbon to this particular audience is a bit like recommending Newton's Principia to a similar audience. It is a work of considerable intellectual achievement, but it isn't a very good introduction to the topic for a modern audience. Not only has a considerable amount been superseded (for example, Gibbon was writing before archaeology really developed and thus had very little access to social and economic data), it simply isn't written in a style best suited to a modern audience. In his time Gibbon was considered an excellent stylist and he is still good for clever turns of phrase, but he comes off as long winded and digressive to a modern reader.

More to the point, while it is still one of the most thorough compilations of narrative history on the topic, its theory and interpretation has largely been considered invalid for some time now.

Decline and Fall is one of my favourite books. I absolutely love the style. The structure and diction of those long, long sentences is like a prose poem at its best.

The introduction to the edition I own describes it as the oldest history book still useful for the study of the subject it covers, as opposed to its historiography. It misses out a lot, and is notoriously unfair to the Byzantines, but what there is mostly still stands as accurate.

I am not a historian but I love this work even if it has been superseded. Gibbon's turn of phrase makes it worth reading just for his use of language.

Gibbon's writing is full of amusing and insightful little gems. I think I can remember the quote accurately, even over the distance of 40 years: "Their lust knew bounds of neither sex nor species."

I would echo many of the comments made above. Gibbon's focus on Christianity as the major cause of the Fall of Rome simply does not hold water. Just consider why the devoutly (perhaps even moreso than the West) eastern part of the empire outlasted the west by a thousand years. A great many of Enlightenment era thinkers had a great bias against Christianity. Perhaps excusable when Western thought had just emerged from the shadow of the Church, but unacceptable and uninteresting today except for historiographical purposes.